Seems as though this might now be a moot point, but no one has mentioned one obvious possibility: Just ask the conductor en route or upon boarding if you can switch destinations. The advance purchase ticket price from Chicago to Portland is exactly the same as from Chicago to Seattle. [i suspect this is true for most other departure points, too.] Of course, there can be a considerable fare difference once ticket sales begin if one leg is selling faster than the other. But on some basic level, going from XXX-PDX is fundamentally the same as going from XXX-SEA. [in coach, of course. Sleepers are a whole different ball of wax]
I actually tried this successfully back in 2008. Due to a change in our travel plans, my SO and I bought a last-minute (well, a week or so in advance) ticket from GFK intending to go to PDX. However, GFK-PDX was literally sold out in both coach and sleeper. (This was right after Christmas). So we booked coach tickets GFK-SEA and then connecting down to PDX via the Cascades. Yes, they were expensive.
However, upon boarding in GFK [at that time the conductor came and took tickets in the station building] we told the conductor of our situation and he just said no worries, go board the PDX section and they'll find space for us. So that was that. We just rode GFK-PDX and skipped going to SEA entirely. Granted, this was not a case of us trying to cheat the system since we genuinely couldn't book the PDX leg. I don't know how flexible a conductor would be otherwise. But if someone is in this situation, I think it wouldn't hurt to simply ask on board if you can ride to SEA (or PDX) instead of the booked destination. The worst thing that can happen is that they say no.