I Agree 100% I tried to get this happening but no one with the skills have stepped up to do it, I would love to see this on WiiU but I haven't got the ability :/
There were some tests a few years ago on Tiramisu but they were never finished. It would probably be easier on Aroma because of the way WUHB is setup like you say, but things would need to be added to the WiiU's Retroarch source too I think to load a game on Core boot automatically. My advice ask around the WiiU Discords perhaps someone will take it on.
At the end of the day 3DS has had the ability to use Retroach forwarders for years so it more than possible on WiiU.