Be nice if you had some more ethernet connections on your router. First option it to buys some more to make this less confusing but if that is not a option then.
There is a way to do this with a single interface but assume 2 for simpler firewall configuration.
Make sure you can define 802.1q interfaces on your 2650.
Create 2 new subinterfaces on the 2650 and 2 new vlans on the 3750. Plug the ASA inside and outside ports into ports defined on each vlan. Define point to point addresses between the ASA and the router.
Place policy routing in the pvc's forcing the traffic to go to the new subinterface that leads to the ASA's inside interface.
The traffic should pass thought he ASA and return to the router and be sent out the serial lines.
If you are running your frame interface point to multipoint... Ie you only have a single IP adderss at the central site that talks to all the remote sites... I am not sure if this will work.
If you have separate subinterfaces you can place policy routing on each.
The router is processing the packets multiple times now so be sure you do not overload it.