Is it possible to develop an app that can make/start a (GSM) phone call without being forwarded to the actual dialer/phone application?
The purpose would be to provide on-screen information to the user along with the call-in-progress.
Nope, to use GSM, it is not possible. You will have to direct that to the dialer. I believe you can develop some sort of solution, if you jailbreak, not sure if that is what you want.