Having my own chronograph, I've tested the velocity of many different kinds of 10MM ammo shot out of all sorts of different barrels and lengths. The lost velocity in a G29 compared to the same ammo out of a G20 is measurable - with some loads more than others - but it's such a small difference that I wouldn't let it bother me at all. The exact load you use will make a MUCH larger difference than the barrel length will. G29's usually spit them out about 50 FPS slower than a stock G20 will, and a 6" hunting barrel in a G20 only gives you an extra 50 to 75 FPS over the stock barrel.
Actually I'd suggest you forget the whole thing and keep your G29 as small and concealable as possible. You've already made the biggest difference you can easily make just by choosing a 10MM instead of a .40 Short & Weak.