How about buying *one* 10-round magazine, and the rest 17-rd magazines. Use the 10-rounder for the boxc check and for the initial load (in the holster), and the other mags for the reloads from your belt?
Your suggestion is not acceptable. It's not within the rules to do it that way. The rules state that you must use the mags you start the match with for the entire match. It is certainly not in the spirit of the IDPA rules. Some local clubs may let shooters do it that way, as the match director for our local club, I won't do it that way.
The SA 17rd mags are $20 each. The true "factory" mags are way up there. Mec-Gar 13 rd mags are $40 each. Mec-Gar 10rd mags are $15-16 each.
I've never tried to fit my Hi Power with the 17 round mags in the IDPA box, because I've never been asked to. The 17rd mags don't stick out of the Hi Power any further than a Wilson 8rd mag sticks out of a 1911, so I don't think you will have a problem.