Is there a way to get a formula or set of parameters for a random npc's physical appearance? Say I want to make my character's face look like Martin or Jeoffry or a Mythic Dawn Acolyte I just slain, what would be my options?
This mod add numbers near sliders on character creation screen,if you know the values this makes them easy to remake.
Unfortunately there isn't any good list of NPC appearance codes online,you will have to find them out by playing with game files.
This is the official oblivion editing tool I would start here and check for values in desired NPC's properties,details...
Wrye Bash is another powerful tool for editing oblivion files,it has some face transplant option by itself but you should be looking just for values.
This and this are 2 more things that could be of use to you,both of them use savegame files to copy the character face,you could get some hints where to look for values or even manage to directly copy npc file.