Hm. Even a Dura Ace 7400 unit looks as though it doesn't extend to the rear/inside of the FD tab/seat tube the way the tricolor derailleur I have on there now does...in pictures on the internet, at least. The limit screws appear to be more in line with the bolt that attaches to the clamp than offset back/inwards.
A 600EX braze-on FD I have on another bike also looks like it doesn't crowd the same area to the extent that the tricolor does.
Thanks for the info/suggestions thus far - I'm sure I'll figure something out here, hopefully simply an alternative FD (in which case I've got my fingers crossed that I can get it right with one strategic purchase/trade).
The DA would be ideal for this build, if it allows a bit more clearance, as would a Chorus/Record braze on from the 8sp era...assuming they don't present the same issue. I wish I had more spares on hand, but not a single braze-on FD in the bin right now. All clamps!
I may have to stick with 23mm tires, but as is, the tricolor FD on there now is *already* a bit close for comfort - so I think I'll switch out either way.