Absolutely. The important thing is how cool the army will look. Flagellants, a couple warrior priests, some templar knights, a tricked out war altar, perhaps some sisters of sigmar as greatswords, or a big angry horde of free company. You could make some REALLY cool unit fillers too! It sounds like a really fun, and creative project. Also, flagellants can be really easy to paint thanks to devlan mud washes.