Heads-up on this, I tried it. I pinch hit for Cishek with Iwakuma. The game gives the bench SPs 0 Energy because they're idiots. So, I had Iwakuma come in with 0 energy and get an out, then give up a double. Bailed him out with my remaining RP (then lost the game on a WOHR). So, you CAN'T sub in SPs as pinch hitters and expect to use them. Screw you if you go into a really long Conquest game, enjoy getting hosed.
I have since added Chris Devenski to my bullpen because of his great Stamina. Haven't needed it, but I'm being proactive in that respect (not getting screwed again before I address it).