I was looking for a way to use emulator/cartridge-dumped GBA savegames on Wii U VC and ran into a whole lot of 3DS related and almost-what-I'm-looking-for guides with loose ends but to my surprise nothing that was specifically addressed to this scenario. After spending some time puzzling I eventually found a way that worked with most of my GB/GBA games (Golden Sun 1&2, Metroid, Pokemon Ruby/Emerald and also Zelda Oracle of Ages/Seasons, to name a few) and thought sharing it wouldn't hurt....
Using your GBA savegames (.sav) on Wii U VC (.bin)
Using your GBA savegames (.sav) on Wii U VC (.bin)