Anyone feel like adapting the rules from SR3?

I don't know.

Here's how I've always seen metaplanar quests: Mages need more Karma than most other characters. Metaplanar quests are adventures only mages can go on. So while everyone else is taking a 3 week breather to heal and wait for their gear to come in, Mr. mage goes on a Metaplanar quest. Lets say it's to find a new nifty spell formula. OK, he goes out and does it, overcomes the obstacles, etc. He finds the spell formula.

He gets the Karma from the adventure he just went on. He uses it to pay for the spell.

The end.

Hmm. As a GM I'd prefer letting the mage metaplane quest for spell reduction than metaplane quest for more karma.

Yes, but letting the magician do a solo adventure, aka an astral quest, requires no new rules. It also allows the player to spend the karma however he wishes.

the difference is that a new spell doesn't cost the spells force in karma anymore, it's just 5, so it's not so pricy. I wouldn't allow it personally.

Hmmm... Yeah, I'll just let them metaquest.