The PJ can be used standalone. That's how I have mine setup now. It's got lots of connections on the back: DVI, 2xComponent, S-Video, Composite, and RGB (PC monitor).
Check the Optoma site for the manuals for details and to see if RS-232 is required. My quick read seems to indicate this is for an input PC connection to either the scaler or PJ rather than something the scaler uses to communicate to the PJ. The PJ should sync to whatever the scaler is sending over the HDMI - in fact, you should set the scaler output to 720p to use it to do all the work and let the PJ just display the pixels it sends.
The Optoma web page even list this feature:
"Single cable connection between projector and image processor eliminates the need for multiple cables, simplifying the installation"
To me, this means all you need is the HDMI-to-DVI cable from Scaler to PJ.