As noted, I think it will depend critically on which version of the drive mechanism you have. I have a 512k with the original 400k drive internally, and I have two 800k external drives. One is the "real" Macintosh 800k external, an M0131:
And the other is the later "Snow White"-styled (is that right? whatever the IIgs is) A9M0106. (
Reference page for Apple floppy drive models.) The MacOS ROM at powerup checks both the internal and external drive for a bootable disk at power-up so booting from an external drive is a legit thing to do; however, if I attempt to boot from the M0131 I get a divide-by-zero error
as detailed by this post on Big Mess O'Wires' site. However, the later A9M0106 works fine. Apparently Apple modified later mechanisms to add a little "slop" to their tach output in order to work around this bug, presumably so they could work as "universal" drives even for Macs that don't have a ROM upgrade.
So, yeah, it will likely depend crucially on the *exact* age and revision of your floppy drive mechanism. Some will work, some won't.
Honestly... I'm not sure what the point of downgrading is. Having the 64k ROM is a novelty, I guess, but I think you can count the number of programs that are actually rendered incompatible by having the 128k one on one hand, and I believe that even in those cases the root cause is usually copy protection schemes that rely on playing evil tricks like varying the disk PWM speed to create freaky non-standard disk formats; anything that tries that will be broken by having the 800k drive mechanism because they self-regulate the speed and ignore PWM, even if you do have the 64k ROM.