What part of the upper transmission (T5) would have a gasket/seal in it that would leak fluid from the top the the transmission housing, possibly around the lower shifter boot? What should I expect to find if I start at the top, and disassemble where the shifter passes through the floorboard...?

Thanks!

 

From what I can remember when I took my tranny off, the transmission fluid is free to come up to the bottom of the plate on the shift lever assembly internally. So, since there is a gasket between that plate and the tranny, if the gasket (actually just goop) is cracked, the fluid could be leaking out there.
To get at this, you'll most likely have to pull out the whole center console, unless you are really nimble and you can get the boot bolts out without doing this. Then just take the shift lever assembly off and check the gasket (goop) to see if that really is what is leaking. Then just buy some make-a-gasket, and put the shift lever assembly back on. Make sure that you clean all the transmission fluid off the contacting surfaces before you apply the goop, otherwise you'll end up with the same problem.
Also, it might be wise to check the tranmission fluid level. It may be too high (doubtful) which is causing the leak, or it may be too low (likely) because it is leaking. It should be level with the bottom of the upper fluid plug on the tranny.
This whole procedure is really simple, not too much to screw up.