If RDS won't support the extension, it might as well not exist. Well, almost might as well. I'm lucky I don't need sfcgal but if I did I might give up on RDS.
I ran into this quite frustrating limitation recently twice in one day, for two different tasks. The MongoDB foreign data wrapper for a legacy data access task, and the rational numerical library for storing accurate fractions for a project dealing with financial data containing multiplicative rates and times where the cumulative floating point errors were throwing off final calculations.