No doubt about it, this is a painful situation.
If you are lucky you might be able to put in rules like any -> destination set of subnets, if the destination set is not too large.
If the branches weren't deployed yet sometimes it is better to allocate VLAN"x" subnets from a larger supernet, like 10.x.branch.0/24.
But on the whole, there is no nice Cisco Meraki solution for this situation.