"I realize I won't have access to the FRS channels unless I bring along a handheld"
FRS and GMRS mostly share frequencies - I think you might be referring to how many GMRS mobile units exclude the low power channels (8-14 if you're counting all GMRS/FRS frequencies) but that's solved by just not using those channels - the group has plenty of other channels to choose from usable by any FRS/GMRS radio.