After I finished replacing the capacitors in this guy, I assembled it with the second floppy drive in place, connected with a self-made ribbon cable. The drives stack very well and are firmly in place. The upper drive could slide around if I forced it hard enough, but it is held in place with significant amount of metal squeezing metal friction.
Unfortunately, I am currently experiencing some undocumented sad mac action; this may be due to attempting to boot from a 9V battery by not having the battery cover on. My PowerBook 100 power adapter is dead, so I'll need to explore alternate powering methods. I have a replacement battery I'll be wiring up to try out tomorrow, but I don't have a charger for it yet.
By the way, if a PowerBook 100 adapter were available to me, could it be used to charge the Macintosh Portable without causing damage? I'm not familiar enough with the charging circuitry on the logic board (or possibly in the adapter) to say. It would also be nice to verify that the power adapter is a switching supply (as it feels pretty lightweight to me) or something else.
Thanks for the information, comrades. I enjoy learning about these wonderful marvels of engineering. Your help is much appreciated.