I was experimanting with LPR and IR/UV cut filters recently and it occured to me that these filters reject photons that are not good for the image but could be used for guiding. I was thinking about a a beam splitter - but for guiding it would look like a flip mirror system with two light paths the imager would recieve the reflected image (front surface mirror so no abberation) and the mirror would pass the IR/UV to the guidecam.
This avoids the issues with guidescopes and OAGs....of course the mirror would have to be very high quality (but then diagonals are these days) and there would be some loss of signal.
So far I have only found one person who has attempted this:
http://www5f.biglobe.ne.jp/~kztanaka/onaxis.html
It would seem the problem is that no mirror with the required characteristics (pass uv/ir reflect the rest) is currently made.