I send ProPhoto to my iPF6100 all the time, and, as a matter of fact I seem to recall that somewhere in the Canon documentation it warns AGAINST using sRGB, because the printer's gamut is so much wider than sRGB. One place where the "stick with sRGB" advice might be coming from is some of the smaller, consumer-oriented (4 or 6 color) printers - is the original poster's printer an iPF (or a Pixma Pro, which would also have a wide gamut)? Since the Canon users on this forum tend to be iPF users, we may have read iPF into the post where it's not stated - if the printer is, in fact, an 8x5x11 inch IP4600 or something similar, the Canon tech's advice makes perfect sense (and a tech who works with the consumer printers may not even have considered the iPF or Pixma Pro lines when saying "all of our printers are made for sRGB"). If the printer IS an iPF, though, the tech's error becomes a serious one.
-Dan