hi,
i can't say i have really been using the matroska container apart from a few file size comparisons with my previous container of preference, ogm.
the reason was mostly cause i hadn't looked into matroska deep enough, until recently that is, when i found out that
matroska is really cool! really great work there devs
as for the tags:
i really like them and would use as much as i could if it was a bit easier to use them and the players had better support for them,
hence i voted #2
breaking scec compliance is bad, but i think you have a pretty good reason for it and the side effects aren't really that important
+ if you should ever do such a thing, now is the time for it since matroska is still quite young