"what looks like an iron cross in there to boot"
Well, as with most issues, I'd defer to Rich on this, but I don't think that would qualify as a true iron cross. Supporting oneself on extended arms pressing against a solid surface uses entirely different muscles than supporting oneself in that position on hanging rings.
The former is challenging, but not unreasonably so, especially if your palms are below your shoulders. The latter . . .
Not in this lifetime.