We carry them on the airbus all the time, (big national carrier) if they won't fit in the overheads then they can go in a wardrobe, failing that we've had them on the flight deck.
Not all airlines are as flexible though with odd size stuff, give them a ring and see what they say. I suspect they will do everything possible to get them onboard with today's disability legislation, if they don't they have the added complication of ordering you a wheelchair.
The ATR-72 has the hold in the forward section of the cabin, just behind the flight deck so they would fit in there no problem.