Here's the thing though, and I mean this with all due respect to the OP: *have* you actually ever sent HD video over actual MMS?
Remember to ALWAYS dig deeper into how things really work, because otherwise you're just setting yourself up for defeat or, worse, to be led around by the nose by someone else instead of being able to provide for yourself.
iMessage, as has been pointed out above, is actually an IM app, not an SMS/MMS app. It can SMS (perhaps also MMS?) but this is only used as a fallback. Otherwise, using iMessage, it will ALWAYS try to send it as an IM.
Incidentally, you should really take a look at MessageMe for Android. If there was ever a community-generated app that Google should consider including in baseline Android, that would be a darned good candidate. It's solid, works well, lets you do all kinds of things that iMessage does, it's free, and all that's required is the other person have it as well. I use it almost exclusively, and frankly the only time I fall back on the regular Messaging app is for those people who don't have MessageMe or my friends with MessageMe who use CDMA carriers and therefore cannot use cellular data and talk at the same time. Me being a GSM user, that isn't a problem.