Just got my 6x Juiceboxes from Overstock last week. Interestingly, the carts they come with are just dummies - no ROMs at all. I suppose that means I'll have to hack directly on the main board instead of going through the cartridge slot. No biggie...
Anyway, what to do about SD/MMC? I found brand-new SD/MMC sockets at Mouser for $1.70ea. No minimum order:
They've got a variety of these in different standoff heights, but the prices are all about the same.
Also, someone said that you can't buy xD sockets without being a member of the consortium. That may be true, but Mouser sells a combination SD/MMC + xD socket for $7.70ea to anyone - I've got one right in front of me. Look here:
It's got two separate sets of contacts in it, one for each type of memory card. Because they're both accessed from the same socket you'll need a separate SD/MMC socket to build the full internal override system, but this will get you there without soldering directly to the xD card or tearing apart an adapter.
Now to fire up that soldering iron...