If there were 100 alt chains merged mining with bitcoin, bitcoin could be adapted to save space in merged mining.
All chains could share a merkle tree as maaku describes here:
"We've eliminated the master/alt chain distinction entirely. The mining header contains a Merkle tree of block chain headers, one for each crypto-token system. This allows infinitely many crypto-tokens to be merge-mined without affecting performance or requiring embedding portions of the master chain into the altchain."
http://www.freicoin.org/implementations-details-and-bounty-7-3-btc-t13-10.html#p158
But I don't really think that's a priority for bitcoin right now (well, your numbers kind of prove it).
I don't think that what bothers kano is not the extra data, but having other currency with the same difficulty as bitcoin's. You know, like Vladimir that sees merged-mining as "parasite the bitcoin chain and destroying the 21 M limit".