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I can add a little info.
For the last few years I've been using a modified Vixen flip mirror - mirror removed and replaced with a beamsplitter plate, to guide my spectroscopes.
Depending on the quality of the beamsplitter you can get some image distortion ( and multiple images) in the guide port. This is not a major issue - PHD can always find the centroid and maintain excellent guiding.
I also use Al's reticule V3.2 overlay to define a "virtual" slit in the guide camera...dropping the target star on the "slit" guarantees that the star lies exactly on the entrance slit. Majic.
HTH
(Details of the mod in "Astronomical Spectroscopy for Amateurs", p 220)
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