3.1 Roms for the A1200 are easy to find, cheap and very easy to replace.
Having said that, i am sure it is very possible as all relokick does is load the 1.3 ROM into a portion of the memory.
I remember the Blizzard 1230 had a program called Blizkick (or something) which would load a Kickstart 3.1 rom into my then Kickstart 3.0 A1200 and i was able to run OS3.9.