[exim] Forwarding to destination / SRS / SPF

I started using Exim several months back, as a means of implementing SRS.
Currently any domain that publishes SPF records I will re-write the sender
address as per SRS rules.

It works fine, I built a file that looks something like this for every
domain I have.

dns:aspmx.l.google.com::25:alt1.aspmx.l.google.com::25
payments:aspmx.l.google.com::25:alt1.aspmx.l.google.com::25
postmaster:aspmx.l.google.com::25:alt1.aspmx.l.google.com::25
r:aspmx.l.google.com::25:alt1.aspmx.l.google.com::25
sales:aspmx.l.google.com::25:alt1.aspmx.l.google.com::25


The file lists every email address and the delivery servers & ports to
forward the message on.

I now want to add another criteria. I want to include the option
(associated with every user) if I should use SRS or not. In some cases the
next server in the email line does not do SPF checking, so in those cases I
don't need to be messing around with the sender address.

I currently read the values of these files in my exim router and associate
the parts to local_parts and route_list.

Can I have exim use the data read from that file to make decisions? Maybe
something like this:

dns:0:aspmx.l.google.com::25:alt1.aspmx.l.google.com::25
payments:0:aspmx.l.google.com::25:alt1.aspmx.l.google.com::25

So could I get the router to check if that value is a 0, do not use SRS?

Am I totally confused? I feel totally confused.