ScreenSpanningDoctor, the application that enables extended desktop mode on some Macs, isn't that sketchy. All it does is set one value in NVRAM to enable it. It doesn't push the video card any more than the a comparable GPU as most GPU's from about 2001 and newer are designed to be able to drive at least two displays. The reason why Apple didn't enable it is because at the time it was a pro feature that was used to sell people on their pro line. G3 iMacs are not able to mirror their display with ScreenSpanningDoctor. They use an unsupported GPU.
I researched this application some time ago, and maybe I should go back for a refresher to see if anyone has had troubles with it. Although yeah, I am sure it's fine.
By the way, did you mean to say that the G3's GPU cannot support desktop expansion? They already do mirroring. Having a display port on that machine was odd... I suppose it had its very unconventional uses, if any.