Sorry, but I’m just seeing your post now. I have made pannatone, and babka using sourdough starter. What I do is substitute the milk for an equal amount of WHOLE milk powder because of the water in the starter (and if the dough needs more liquid, I can just add more water). Because of all the sugar in these doughs, I boost the dough with about 1/2 tsp. of instant yeast. I basically use the starter for flavor and better keeping properties, not necessarily for 100% rising ability, so I don’t see a problem with using a bit of extra yeast.
Also, most of these breads (especially pannetone) require a sponge anyway (which is usually flour, water, and yeast), so subbing a sourdough starter is easy. You can just sub an equal amount of sourdough starter by weight for the sponge in the recipe. I would still recommend boosting the dough with a little bit of instant yeast.