I've been fiddling with that but with little success before. Some sources claim that ASA will never send icmp redirects, which is necessary to let the other clients on inside know that there's another gateway for the specific destination, while other sources claim that ASA up to 8.2(1) will not do it, implying that newer will do. IIRC, I tested on 8.3 with little success.
With a little luck it'll work and maybe I just gave up too early. You will definitely want the same-security rule:
same-security-traffic permit intra-interface
Make sure there's no funky nat, and that your access_in acl allows it.
I ended up sticking the particular device (in my case an openvpn router) on another (sub)interface to achieve the routing. Don't forget to report back on your success or failure!