This will be ugly. :-) You could do this:
remote sites
|
internet----2611----\
| |
pix----/
You can policy route traffic coming into the 2611 from the remotes and destined to the internet out to the pix, and let the pix route it back in and out to the internet. You could also policy route the traffic off the internet to the pix, and then let the pix route it back to the 2600 for routing towards the remote sites.
Like I said, this is ugly.
Russ