Comments solicited: assignment of non-routed network numbers

We have recently received a number of requests from customers wanting IP
network numbers which are never intended to be routed to the global Internet.
How are other providers dealing with requests of this sort? In the absence of
network numbers which have been explicitly set aside for this purpose by the
global Internet registry, I can't see a good way for a provider to allocate
network numbers for this purpose without having them needlessly consume the
provider's address space. One compromise might be for the provider to reserve
a block which can re-used by more than one customer, but that block will still
consume a chunk of the provider's space. Also, how large should that block be
to satisfy all requests for non-connected network numbers?

Your comments on this matter would be appreciated.