Yes, the water you used to hydrate dried mushrooms is a valuable commodity for sauces and soups.
Interesting that you prefer not to use stocks.
Interesting also that the mushroom growers' marketing campaign ahs been successful enough to push the idea of "baby bellas."
Without any sort of correction intended -- just saying is all:
There's no such thing as a "baby 'bella" or baby portobello. If you want to get all curmodgeonly and proper English languagey about it, there's really no such thing as a "portobello" either. A "portobello" is not a separate species or type, but just a very large "crimini," aka "brown mushroom," which is. So, a "small portobello" is actually a plain jane crimini. "Portobello" as a name is just marketing hype. It's a way of selling mushrooms which otherwise could not be sold; not only because of their size, but because the gill structure has opened up.
On the other hand, you could argue (and correctly so), that English is a living language, and what begins as a term coined for marketing purposes becames legitimate through use. If enough people say it, it's a word.
Plus, as it turns out, their size makes portobellas amenable to a few techniques and uses to which smaller mushrooms are not.
It's probably a function of age, but "baby bella" draws a reaction out of me.
Cranky old BDL