You CAN get things onto the memory card, BUT you can't launch them on the GC.
EG: You can use a GC mem manager to put modded save files on, or mod the source to put other things on using the Wii, but then when you put it in the GC you can't launch these things without an exploit to do so, so there is no point. I myself would love to see this happen one day as I have a couple of GC sitting gathering dust and I wont put in a mod chip unless it's non solder and I cant find a non solder GC chip.