My point was not that using alternate codecs was technically impossible, but that it was
de facto infeasible — nobody does it (because you'd only want to use it if you already wanted to use a free codec for it's freedom), as such nobody tests against it, so it won't be well supported by implementations, not even to necessarily give you a 'codec not found' error message.
You're right that OGG and native Flac are different containers, with differently stupid approaches (aiming too high and too low, respectively). The later Flac formats do specify OGG's broken metadata system, one of several reasons that nobody ever distributes Flac files with any internal metadata. I've never seen a Flac-in-OGG file in the wild, but I imagine that seeking is at least as broken there.
Seeking in Quicktime works just fine for all manner of esoteric codecs and containers — the problem is with the FOSS stuff where they decided that they'd do something inventive, and ended up with something pointlessly different — enough to make it annoying to integrate into any existing system — and no better than what everyone else does.
Sometimes what scratches your own itch and is fun to implement is no good for anyone else to actually use.