My new Corbin modular seat came today. So I have determined now with the Corbin seat, Phil's rack, and the GIVI E55 case, that there is no way to get the Corbin backrest to work. I will need to get the GIVI backrest, which I'm sure is not as nice.
I guess for me that has been the only downfall of the combination that I have. Phil's rack works great, looks great, and really does feel so much more solid than the stock rack, and we have no issue with insufficient passenger room with the combination.