I did more testing. Everything's fine. I'll provide details below in case they're helpful.
I have a Logitech K400r wireless keyboard. Its Fx keys (F1-F12) default to media/web functions, so to send a plain F11 to my computer, for example, I need to press Fn + F11. There is no physical switch on the K400r keyboard to restore the old/sane behavior where the Fx keys send Fx keystrokes, but solaar has the ability to change something in the keyboard's firmware and fix this for me.
I can confirm that solaar is working fine for what I need it, even with both libffi.tcz and libffi7.tcz in the dependency tree. I can toggle the old/sane Fx key behavior on and off without any problems.