BTW, Rearden (apparently from Jeremy?) pointed out that my simplified CTV-like scheme was flawed because it didn't commit to the order of input txids.

You need to xor SHA(inputnum | intxid) for each input to fix this.

I still like the scheme, because it clearly commits to everything the txid commits to (with modifications required by efficiency concerns). Like a "forward txid" to mirror the normal txids which are backwards references.

I should write it up, for comparison with CTV. Maybe once I've done that I'll no longer think it's a significant simplification?