I like that, I was experimenting with !TMPL_VAR but yours seems more semantically appealing. (I'm practicing my impressive technical speak there...)
Using !TMPL_VAR would allow for "3-pass" or "n-pass" templating. That
is, change !TMPL_VAR to TMPL_VAR on each pass. If you then have
!!TMPL_VAR, it survives to the third pass.
I actually do this (albeit with a different templating system), though I can
recall only a couple of times I ever needed !!.