anyone know how to change senate missions?

was wondering if anyone knew how to change specific senate missions and what rewards are what. I've looked through the descr_senate.txt but its not very helpful. Specifically i was looking for a way to increase the likelihood of receiving first cohorts in the campaign map. I've played through two complete campaigns and 3 15-30 province games with roman factions and am as yet to receive an eagle. I've never seen one playing with non-roman factions either.
I'd also like to be able to mod senate rewards directly as i can think of some rewards and requests i would like to add in that are slightly less generic.

Some of the these things I think can be found in the desc_...something with senate in its name...

There is at least the likelyhood of getting a mission...and perhaps more...I only skimmed it...

Great if you find the one to give eagles since I have neither seen one and I've played 1 long campaign (brutii) and short with both the others as well as most other fractions...