[exim] Making $pipe_addresses a real variable?

Hi all,

I'm increasingly tied up in knots trying to run a transport_filter in a
lmtp transport conditionally, depending on which addresses have been
batched up to be processed by the transport. I don't want to fork the
pipe process at all if I don't have to - but the only way I can tell if I
need to is by inspecting $pipe_addresses, which isn't a real variable. If
I do run the filter, though, it needs all the addresses delivered as a
batch (whereas if I push the conditional into a router, there's no way to
propagate the result to the lmtp transport).

Might there be any way in the future to make $pipe_addresses inspectable
within string expansions in a transport?

thanks,

Matthew.