Very interesting! Incidentally, I have just read A J Hall's "Time Shall Not Mend", which is a HP/Lois McMaster Bujold crossover, and so good. The author managed a really impressive pastiche of Bujold's quite distinctive style. It was also a good example of something you mentioned in your article: where two sets of characters are in fact in the same universe without realising it(just, in this case, a thousand years apart). Pym as a secret Wizard was both amusing and alarmingly plausible. The only thing I didn't like was the nastily misogynist characterisation of Ellie Quinn, which was unnecessary and unpleasant, but that was irrelevant to the crossover aspect.