Hmmm... I was pretty sure that somebody figured this out before... and that what I suggested, could elaborate upon it by a few simple hacks.
You might remember from back in the day there was an event trick to color the text.
How to change text color:
I don't know the details for it but apparently this command can change the color of the text in the dialogue boxes:
MemCpy2E(43, 09, 05, 01, 00)
This one makes the text yellow, but other variants are possible.
(Now known as "ColorMath", under Scene Manipulation. Don't know the equivalent parameters under the revised command description yet.)
But that changed
all the text at once. It appears to alter the palette data directly. The text is still sharing one palette. I think the SNES can't update VRAM (including the palette) on the horizontal render, only vertical (by scanline), and that still requires some tracking to time it right.