The problem is more that PureGDK's render loop is closed off within the DBPro engine's 'sync' function. I can't find a WinAPI/DirectX function that would set a callback to be called right before DirectX stops rendering to the backbuffer.
If I can find a way to get Matrix1Utils to work(just found the video that explains how to use plugins), the M1U_PostCameraCallBack() would be ideal.
EDIT: Just checked, the correct DLL is being loaded(Matrix1Util_26.dll) and other commands do work properly.
EDIT2: Solved, with no undue amount of head smacking on my part. It seems that calling CEGUI's renderGUI() function after the dbSync() works just as well as injecting it directly into the render cycle. While a render callback would be nice, I no longer really need it.