You can specify the subnet id part since 7.21, no need to specify the full address. However there is one exception, that is the first subnet (the one with subnet id all zeros) cannot be specified that way, that one will be dynamic. You can read about this in the 7.21 threads.
Example: ISP gives you /56 prefix, that you put in isp-pool, under /ipv6 address, you can assign hardcoded prefixes like this, together with from-pool=isp-pool:
address=::1:xxxx:yyyy:zzzz:wwwaddress=::2:xxxx:yyyy:zzzz:www- ...
address=::10:xxxx:yyyy:zzzz:wwwaddress=::11:xxxx:yyyy:zzzz:www- ...
address=::fe:xxxx:yyyy:zzzz:wwwaddress=::ff:xxxx:yyyy:zzzz:www
And they all apply correctly and the interfaces will have fix assigned address & prefix from the pool. Except for this:
address=::00:xxxx:yyyy:zzzz:www, which is identical toaddress=::xxxx:yyyy:zzzz:www
That entry with subnet-id 0 (00 in case of /56, 0 in case of /60, 0000 in case of /48) will always be dynamic, and the prefix will jump to the next available subnet-id every time the DHCPv6 binding renews, or every time you make change to the interface. If you want your interface to have predictable & fixed subnet-id you should make sure the interface does not have the subnet zero assigned!
If you have a preferred order for your interfaces, that will mean you'll have to shift all your assignments once. If the order is not important, then you only have to edit the interface that currently has the subnet-id once to give it a higher subnet-id.