Thanks all for your input. It looks like we are going to have to keep a standard breaker in place. This application requires a 60 amp breaker and Square D stupidly does not make 60 amp GFCI breakers for Homeline panels (only for QO panels.) I tried searching for a UL classified breaker of this flavor but was unsuccessful. What I may do, although it's not my preferred solution due to double-breakering, is to have a separate 1-circuit load center/disconnect mounted beneath the main panel (where the underground conduit enters) to accommodate the GFCI breaker. (Or, I may just make peace with it as-is, as it is technically up to code, even though I'd argue this is an area where the code falls short.)
