Hmm.. Do you have any examples of such ISPs? I’ve only seen them give out a /64 to the PPPoE / WAN interface and then also have a routable /56 or /48 which is almost always available using DHCPv6-PD. The /64 is only used for communication with the ISP (can also be a /126) while everything else if up to the router to manage.
Although I do get a /48 from my ISP there is nothing I can manage about the subnetting. The 16 bits between my prefix and my networks are assigned by my router using DHCPv6-PD and there is nothing I can do to set these bits. They are assigned to my networks sequentially.
I don't think it is feasible to use those bits with a special meaning.
Rob