You're right but my objection with a front derailleur is it's over-engineered for the purpose, and would be silly if you designed a chain guide that way. Evidently I'm as ocd about some things as other cyclists, just different things I guess.

I wonder about wrapping a chain cover over that and maybe mounted or wrapped to the bottom of the down-tube. If nothing else a strip of aluminum or polycarbinate. Or split some conduit?