Yep, my vote in for investigating OP_RETURN too.  It'll be another couple of months until OP_RETURN is widely supported by the miners, but once we publish some transactions with OP_RETURN and gain confidence the majority of miners will include them (and thus we don't have long lead periods for block inclusion) then we introduce a Class C as you note.

Looking through Gavin's notes he picked 80 bytes to provide for a 64 byte hash of whatever is being stored (eg SHA512) and some extra metadata.  I don't think we need metadata so we could use the whole 80 bytes for Mastercoin transaction data.

I just wanted to point something out though as I think you guys may have overlooked it (sorry if I'm reading your comments wrong & making a bad assumption); the pull that Gavin merged for OP_RETURN is per transaction not per output.  This means each Class C (OP_RETURN) transaction could only have one 80 byte packet and thus a total transaction space of 80 bytes, where as Class B (multisig) as defined in the amendment provides for a total transaction space of 7,650 bytes.  That's a distinction we need to keep in mind - things like 'simple send' would easily fit into a Class C transaction, but some of the larger functions (eg smart property) may require more than 80 bytes transaction space and thus need to stay with a Class B transaction.

Thanks!