John Peel and I briefly discussed, back in the late 90s, the idea of a crossover that would have the Trek cast appear in a DW book for the BBC, and the Doctor and companions visit the Trekverse in a Pocket Books book, but never went as far as pitching a story...
The year John Ordover came to Shore Leave (I think it was 2003) he talked briefly about a Star Trek
project he discussed with Bantam Books in the mid-90s that never got far off the ground that would have been like that. Pocket would have done a Star Trek
novel that told the story from the Trek
ian perspective, while the Star Wars
novel would have told the Wars
ian side. However, it wouldn't have been an official crossover. It would have been more like the unofficial Marvel/DC crossovers of the 1970s where the serial numbers were filed off but it was still clear who the characters were. If you read the two books together, you got the full story and you could piece it together. If you only read one book, you either had the New Republic encountering a ship from a democratic Federation or a Federation starship encountering a remnant Galactic Empire.
Also, swapping characters between publishers and having two books would probably be the easiest approach. It's how DC and Image worked their Batman
crossovers in the early-90s. Each publisher was allowed to do what they wanted with the other's character.
Of course, individual authors can always cross over with their own creations from other franchises - the Feledrin gestalt alien in On The Spot is the same species from the DW audio Unregenerate, for example...
I remember reading your Voyager
novel outline on Usenet a decade-plus ago, with Janeway battling the Master. Err, excuse me. Koschei.