Yes, we did!
We had to hook the uplink computer to one of the regular wired ethernet ports on the router, not the "internet" port, for reasons which I don't understand. Then there were several configuration changes required to the router, specifics of which will vary by router and firmware. Unfortunately, the router we had couldn't be switched to the Openwrt
firmware. That would have changed things. We moved the dhcp and dns servers to the uplink computer, set fixed IP addresses for the router and the uplink machine, and set the router to throw everything to the uplink machine. We used dnsmasq for the dns server, but there are also others available that would work. A couple of dnsmasq.conf file tweaks later, we were on line. We set kppp to reconnect on disconnect, because the uplink connection seemed to have a timeout, and it wouldn't reconnect automatically on internet access attempts. As I recall, we hit some routing issues on the uplink box, with fixes like:
route add default ppp0
ping, traceroute, ifconfig and route are your friends. Start-up of the uplink box can be problematic, because the system detects the wired ethernet interface, but not the USB modem since it isn't active, then tries to use the ethernet for all traffic. The above line fixes that.