I think to do Prefix delegation, you just set a pool with a smaller prefix than the netmask on the block of addresses given to the prefix.
Usually, I would use RA messages to allocate WAN addresses on downstream routers, and then DHCPv6-PD to allocate prefixes for the LAN on those routers (and insert the route to those prefixes into the mikrotik).
But how do I do both at the same time? I need this because the DOCSIS (cable) spec apparently says no RA to configure the WAN of modems -- only DHCPv6. And you need DHCPv6-PD to hand off blocks of global address space. If I try putting two entries in for the same interface, it says that it is not allowed.