Is it possible to make a group collapsible based on the value of a parameter? I can't use parser functions in the group tag, but maybe there is a workaround using the tag function? I tried but without success so far.
There's not. I'm sure we never considered it. But in general, if it's an XML attribute parameter, it can't be changed by templates or ParserFunctions, as those process after the infobox does. Keep in mind, PIs are not markup; they're tag extensions.