When using a reverse mount adapter ring to reverse mount a lens for macro photography, is it possible to use a lens designed for a different camera system so long as it has the same filter threading?
It shouldn't matter at all if you are using a lens that is stopped down manually. In fact, it may be easier to use an older or off-brand lens than the current crop of lenses having only electronically-controlled apertures. And step-up and step-down filter adapters ought to be able to handle the attachment issue (but, of course, it will change the distance between the front element of the lens and the sensor, so you'd need to take that into account when calculating magnification).
There are, or at least there used to be, reverse-mount systems that could control the aperture automatically -- there was a little tethered unit that was essentially a bayonet mount for the lens with a lever that operated the lens's aperture tab. (There's no reason why the same couldn't be done for more recent electrically-driven apertures.) Obviously, the lens would need to be for the right mount for this to work properly. As I recall, though, these auto-aperture reversing rigs were godawful expensive, and they didn't bring anything to the party except the ability to focus wide open without manually stopping down afterwards (and sometimes to meter through the lens wide open). Unless you are in the habit of forgetting to stop down, the difference between a simple, no-moving-parts mechanical adapter and a complicated, expensive, vendor-specific electromechanical system is a lot of money for nothing, so even if it's available, I can't see the point.
Yes, you can use any lens on any body when reversing, as it is the reversing ring that interfaces with the body. I use a 40 year old Ricoh lens with a Pentax K mount on my Nikon D5000, for example. Any filters or adapters that fit on the thread of the lens/reversing ring can be used as well.
See my Photo SE blog post for a complete guide to using reversing rings here.