If a judge can see multiple events, the canonical way to do that is to use a single combined judge category. If that's not desirable in your situation (invoices, etc), then there's not a much workaround than the types of things you've experimented with that I'm aware of.
If I was pushing a ballot to a judge outside their normal category, I'd probably either use their judge info screen to temporarily move them to the other judge category, create a duplicate judge record for them in the second category (if I anticipated the issue continuing), or just use the "force any judge" option on that particular section.
At least in the past, I'm not surprised you've run into issues with your "super pool" idea - the pools weren't really designed to be cross-cutting over categories like that,
I don't know much about that new option, though - Palmer would have to chime in here if that feature is done and production ready. Just based on the description, it would do the opposite of what you're describing, and prevent a super pool from working at all - though it implies that if that's left unchecked, you might get more mileage out of the idea than you used to, as presumably the default is then to let the judge pool cut across categories.