Your approach won't work-- OP_ADD is limited to 4-byte operands. Changing that would require a "hard" blockchain split / entire-network-upgrade.
I wish to submit the following draft BIP for discussion and possible
formal BIP review.
https://en.bitcoin.it/wiki/BIP_M_of_2_DRAFT
It is a proposed alternative to BIP 16 ("/P2SH/").
Thank you,
Ben Reeves