It took some time, and since my little tanker have been to Molten Core too many times I honestly was surprised he was not already exalted to the Hydraxian Waterlords. But finally tonight I spotted a few guildies who were going for the Molten Core Achievement.
Only needing 290 more reputation I got a bit worried when I was stranded at 20999 after downing 3 bosses (who grant 100 reputation each). However it appears that only Golemagg and the big mighty Ragnoros himself grant the last bit of reputation needed.
As a bonus I also got the at the moment rather embarrassing achievement 10 Exalted Reputations.
Well, what can you say. I'm not a grinder :)