Well... if they have the soldering iron, the basic ability to orient
the polarity of the (1N400x or zener) and heatshrink it (or whatever
covering), yes I do feel if they have these basic skills and any parts
lying around, 1N400x diodes are as common as dirt and almost as cheap,
as are many resistors.  Look at all the electronics you have in your
home, I meant retail products.  Count the # of > 1W zeners in them,
I'll bet the # is low if not zero.

You are fortunate to have such a well stocked Radio Shack then, they
dont' even offer over 1W zeners online.  Sure, Digikey and Newark, et
al do, but if you're going to order from a big electronics house like
those then it makes all the less sense to use a zener.  Voltage
reduction is not the actual goal here, it is merely assumed (by some)
that it is the means towards the end of reducing fan speed (mostly to
combat noise).  Current, not voltage, limiting is the more effective
way to do this.

Well you ARE the one who brings up Radio Shack or Digikey/etc.  If
you're going out to buy the things then a 15 cent 500mV zener and 1A
pass transistor is better than a 5W zener and costs less too.

Given one of enough current capablity to be called a rheostat, it'd
work, but is a personal decision and I can only give the reasons why I
wouldn't.

-  More expensive
-  I don't need to change the fan speed, it gets set right at first and
never fiddled with.  Fine-tuning down to a dozen RPM one way or the
other is pretty insignificant, IMO, and might not be obtainable anyway
on the wire-wound types since there is an inherant granularity to the
adjustment when it hits each wrapping of the element wire.
-  Another mechanical part subject to failure.  Remember my ideal is 50
years (even if not really used that long), and it's quite clear that
pots have potential to degrade in far less time than that, especially
if a marginally-current-rated "pot" subject to thermal cycle wear as
well as the other more obvious mechanical issues.
-  You need a place to mount it, and it can look a bit ghetto unless
you have a fancy knob and even then it can look out of place.  The
simple inline resistor has unde 1 inch of non-flexible segment in the
fan lead and can be completely unobtrusive.  I've used them for many
years and they work great.
-  100 Ohm can be a bit on the low side.  I typically buy
manufacturers' "low" speed fans to begin with and they may need a
little higher than 100 ohm, particularly on larger than 80mm fans.
There are ways around that if one had the rheostat, having a couple
diodes in series too, but unless that adjustability is needed the time
to do it goes up, as well as cost, size.  At lower cost than such a
rheostat is also a linear such as LM317 plus one resistor, in
current-limiting mode.  That is a bit more involved though, and why I
seldom suggest it as it has no particular benefit over the higher
wattage resistor alone, actually less of a benefit because of the vdrop
of the regulator being part of the power loss to the extent that the
resistor used with the LM317 is a lesser current limiter as % of
incoming voltage opposed to using only the resistor.  The difference is
only slight though, could depend on a specific fan whether it mattered
in practice.

I'll bet it doesn't, actually take as long.  Type 1N4001 into one of
the surplus electronics 'sites (else if going with Digikey, you pay the
small-order fee and that $1.42 zener ends up costing $1.42 + $5 small
order fee + (at least) $3 shipping = $9.42.  For about that cost you
can get a few hundred 1N4001 delivered from a surplus 'site or at least
faster since you don't have to pour over the specs as with the zener.
Keep in mind though, I"m not the one who advocated the string of
1N4001.  They will work, but I advocate a power resistor.  You can get
a bag of 100 for about $2 at one of the surplus 'sites, or just spend
the $1 for only a handfull elsewhere.

To put it to the test though, right now I'll go to Digikey...
Enter in search term "1N4001" and you get one page of hits, can just
buy the very first item that appears and do fine.  If you feel like
scrolling down a dozen lines they're a couple cents cheaper.

Next at Digikey I enter "5.1V zener".  Immediately the next page causes
the user to choose package type.  Who, not knowing already to use a
zener and having read your reply, will know the packages they come in?
Regardless, let's support they skip that and choose at least 1.5W for
some reasonable margin.  OOPS!  Those are all surface mount parts.
Hopefully they are experienced enough to realize this, but maybe not,
and either way, this is certainly taking a lot longer than getting
those 1N4001.  So we go back and choose 2W zener, get one choice in
small quantity at $1.42 each.  Ok so far, except we now need to
recognize that it's DO-41, meaning it's heat density is higher than any
other alternative... hook it up to the wrong fan and you will have to
isolate it somehow, because it will get hot.  They're often rated up to
the (n)W with the disclaimer than to handle this, their leads are
soldered into a board to 'sink away some of that heat.  You may find
that a 2W zener isn't even enough, so do you know this ahead of time
(does someone else buying them), or do they now go back to Digikey and
spend at least another $X for the 3W size.  Oops, those are all surface
mount too.  This is taking a lot longer.

OK then, back one page at Digikey to the 5W zeners.  Hmm, they don't
stock any!

Ok, now go to Mouser or Newark or (insert your fav. here), and
lather-rinse-repeat.

I didn't suggest the resistor just for the heck of it, it's an ideal
way to do this _unless_ one must have adjustability later.  I find I
don't and have a TON of systems with the resistors... even having
zeners and pass transistors, 1N4001, LM317, et al here to choose from.
Even so, the other methods will work provided the details (like Zener
wattage rating) are kept in mind.