Two thoughts here:
First, they have three viable markets:
(1) Follow-through traffic onto their other routes. In this respect, the trip would serve as a feeder line into Vancouver, and I suspect they could do a decent amount of business with folks wanting a day or two in Seattle as well.
(2) Cruise-connecting passengers. This is likely bigger than anyone realizes, considering the fares into/out of Vancouver's airport. It's likely that by "training" folks down to Seattle and putting them on a flight out of the airport there, it's likely that RM could pocket a decent amount (say, $200+ per round trip) just on the fare differential, not to mention being able to sell folks on an easier customs process.
(3) Passengers just traveling SEA-VAC (both those on peak days of travel when the Cascades is sold out or close to it, and almost assuredly those they could put a "luxury weekend" package together for...remember, 48 hours out of Canada gets you the full $800/person duty exemption).
Second, on "isolation fares": Right now, the Cascades offers tickets SEA-VAC at $30/40/60/72 (the Ambus is $45), with BC being +$22. I would think that Rocky Mountaineer could probably sell seats at $100-ish each way and still get business (particularly if they're including food service of some sort each way...$100-120 each way with their normal discounting on the round trip should also be high enough to enable them to turn a profit on the run without freezing them out of the market). If they're getting a residual on hotel packages as well, they should be more than fine at that level.