I've not come across DDL triggers before- they look quite interesting and a relatively sensible place to capture changes.

I can see these being helpful for evaluating roll-out accuracy. I think my concerns with this approach would probably start to split hairs, off the top of my head, they wouldn't capture anything that lives outside of a database but within the database server (mail profile and accounts, sql server jobs, linked servers for example), or indeed the databases themselves. Moreover, triggers are somewhat simple to disable.

In our setup, we weren't as concerned about being able to undo or redo changes automatically- I guess we're a simpler setup. Also, our VCS integration is significantly more basic than the one you outline here. Mostly we're satisfied if what's in the baseline matches what comes out the source extract.