This may be a question for @tav, though I’d be happy to hear anyones solution.
The ParagraphPro stack seems to recognize the Foundry Typeface stack as a font source - the DropCap child stack seems to only recognize FontPro font vaults. Note that I’m only going by what shows up in the settings here.
Is there a way to get the DropCap stack to use a Foundry typeface?