Hmm. This sounds like something up TwistedMexi's alley - have you considered reaching out? (I have no idea how one would reach TwistedMexi apart from joining his Patreon, though). I mean, there's already the re-enabled devcheats, Always TestingCheatsTrue, Always MOO, and Always CAS.FullEditMode mods, so perhaps there's a similar way to do that here.... and if not, he might be able to point you in a helpful direction?
If there isn't a way to do this by the method you describe, what about a roundabout method that gives the same end result? Perhaps a script that looks not for the F5 keypress, but instead looks for/targets the (presumed) boolean changed by that keypress and changes it to ON whenever it detects its state is OFF? If monitoring the on/off state at all times isn't possible (or if it would be too demanding/impractical), perhaps have the script check the quarter/half-tile placement state whenever the player enters Build Mode or loads a lot?
Alternately, if a guru does wind up in this thread, this would be really nice to have in the vanilla Settings/Game Options menu. I can't imagine it being difficult or time-consuming on Maxis' end to add it as a checkbox option in the settings; I always forget that this isn't already a setting until I pop into build mode for something & then have to google what button I have to push to enable it. :/
