Would it be possible to use a starter in recipes that call for a sponge? If so, how would one proceed with the measuring out of ingredients, that is, using a fairly liquid starter?
Oh, yeah you can do that. I do it sometimes. What I'll usually do is figure out what hydration the sponge is and then make my final starter build that same hydration, and build it up to the same weight as the sponge. Or just follow the sponge recipe, but omit any yeast, salt and any enrichments, then add a little bit, like 10%, of old starter. You'll get a little extra this way, but you know, you'll need extra anyway to save and feed. If your sponge calls for things like honey or milk powder or salt, I'd think you could just add them to the final dough.