I am trying to figure out a way, preferably without using OBSE, to permanently change a mesh on the player character by means of a script. I have created the mesh which I would like to use, but cannot figure out how to either replace the base body slot mesh on the player without changing the base species or to script it so that when the player has nothing equipped there, it equips, but when the player fills that slot with armor or clothing, it comes off and only reequips once the armor is removed. Is there any way to do this?

Not without OBSE, no.

The set of 4 body meshes (upperbody, lowerbody, hand, foot) every race uses is hardcoded and cannot be altered on a per-race basis. You can only alter these NIF files directly, which will affect "all" races.
The functions to check what's equipped in a certain slot are all OBSE-dependant according to the wiki. The Vanilla game doesn't have such.
While you could force-equip your meshes with Vanilla functions there's no way to know "when" to do that without OBSE.

There are other ways to work around this, but without OBSE I don't even want to imagine how complex it will get. :wacko:

I created http://tesnexus.com/downloads/file.php?id=14326 a while ago that details how to do exactly what you're asking, but I made it years ago so I can't really vouch for the quality of it anymore (it has received some endorsemants and compliments though, and I recall it working perfectly).
It always remained pretty obscure and unknown. It probably would have been much more logical to have just put it on the wiki as a guide