[Exim] Host lists in string expansions?

Hi,

Is it possible to look up values in a host list (or other list type)
from a string expansion (running Exim 4.10)?

Specifically, I have a file containing all our local networks, and
the masks vary on each one (i.e. 1.1.1.1/24, 2.2.2.2/28, 3.3.3.3/20).
This is assigned to a host list called "+localnets".

Now, routers do not appear to have a "sender_host" check option that
would allow me to exclude a router based upon source address.

So, I was hoping to do something like:

condition = ${isinhostlist {$sender_host_address}{+localnets}}

...but I can't find an appropriate function.

The "mask" expansion won't seem to work since I have to know the
mask width ahead of time. Host lists automatically interpret each
item in the host list separately and auto-widen the search address
for each comparison.

Is there any way to do this?

If not, my wish list item would be to either (a) have a sender_host
check item on routers or (b) have some way to look up a key in
a host/domain/address list in string expansions using normal
list lookup methods.

Regards,

Dean Brooks
dean@???