Like your idea of just trying the RD, just try the FD. I would bet it will work fine. Why? Because I've installed two different compact cranks on my mid-80s frame that was obviously made for a standard size crank and both worked perfectly. Not only do I think it will work fine on a bike "designed" for a standard crank, I'd also bet that your frame, if it's fairly modern, will have been designed with compact cranks in mind.

It will probably work fine as-is because the braze-on slot will be long enough to get the derailleur low enough (there is a span of tolerance for FD height after all). Even if it seems to be too high, give it a try and see if it works. Both of the compacts I set up on my old frame worked fine - one a Shimano compact and standard FD, the other a Campy compact and standard FD. Both are a little higher than I have FDs on other bikes set, but it still works fine. I don't measure all that accurately, but my guess is that they're on the high end of spec, while I usually try to set them on the low side, and am used to the FD looking like that.

If it does not go low enough to allow the FD to shift perfectly, I'd bet it will be within a mm or two, and you can file the slot a little to allow the derailleur to go low enough.