I searched but found little on this subject. I unexpectedly got a keiki on a phal several months ago and it's gotten to the size now that I'm about ready to cut it loose, but virtually everything I've seen talks about potting and I'm wondering if slab-mounting could also be an option. Any suggestions?
Of course it can be mounted. When you separate it from the mother plant, it will behave as any normal Phal.
The good thing is that the roots will already be adapted to the humidity of your setup (some ExoTerra thing in the background I suppose?) so it shouldn't be too hard to transition.