I finally got SSBB working, and here’s how. But let’s start with what does NOT work.

First, my setup:
PAL Wii with latest firmware (Bought on the release date)
WiiKey (Firmware 1.9s)

Try 1 (No go):
Enabling update blocker on the WiiKey:
Doesn’t work, from reading a litte on the net I found out that you have to enable dev mode as well.

Try 2 (A bit better):
Enabled dev mode as well as the update blocker on the WiiKey. The disc loads correctly, but when you try to start it you just get a black screen.

Try 3 (Almost there):
Got a tip to try it in 50hz mode. Did that, and… it works. However the game runs awfully slow, and you get audio artefacts from some sounds which means it’s not really playable.

Try 4 (Success!):
Decided to try FreeLoader for Wii. I disabled update blocker and dev mode on the WiiKey, and changed back to 60hz.
First I went to the disc screen and inserted the FreeLoader disc and waited for it to load (you may see some graphic artefacts during this time). Then I ejected the FreeLoader disc and inserted my SMBB disc and started it. And it worked fine.

If you get a white screen saying the disc can’t be read after the “Loading…” message you’re propably suffering from a bad burn.