VIA's Corridor rotation, designed for as much run-through service at Toronto, Ottawa, Montreal includes some trainsets getting 2-3 'legs' per day i.e.. 31-45-87, 84-46, 53-87 and different pairings each day for the Ventures. There are few 'legs' that are served by the same trainset throughout a week, with a few exceptions like LRC runs 67-79/72-66.
VIA's rotation also includes returning to Toronto or Montreal for servicing, refuelling, cleaning, running or heavy maintenance of these trainsets. Not that I disagree at all with your idea. It's just getting VIA to remarshal trainsets to different lengths, whether to evade the CN-imposed crossing speed reductions or otherwise, is like pulling hen's teeth - and they're pretty rare!