We've been thinking of this as about a week on the road, a couple of days in Portland hanging out drinking local beer, then the train back to Vancouver BC and home to Victoria on the ferry - the Cascades train up from Portland apparently allows bikes as as luggage, intact, instead of just in boxes as cargo, which is useful.
We'd have to extent the trip at least a couple of days to include the Mountain Loop Hwy, but that would be an interesting detour.
Going by the campsites I found, one possible itinerary (not including a Mountain Loop Hwy detour) would be:
Day One - Victoria-ferry-Anacortes/Bay View SP 60km (+ferry time)
Day Two - Anacortes/Bay View SP-Tolt-MacDonald Park and Campground - 135km (!)
Day Three - Tolt-MacDonald Park and Campground - Lake Easton SP 99km
Day Four - Lake Easton - Yakima 128km
Day Five - Yakima - Brooks Memorial SP 92km
Day Six - Brooks Memorial SP - Starvation Creek SP - 121km
Day Seven - Starvation Creek SP - Portland - ~93km
This makes Day Two the longest day, although all of Day Three is one continual climb until just before Lake Easton SP. Day One is shorter than it needs to be, but there's a long gap between campsites between the area right around Anacortes down to the four campgrounds Snohomish County runs near Arlington - 50-70km, which given the ferry only gets to Anacortes from the Canadian side around 1500 puts you awfully late to be getting a site at one of them, although the county does take reservations.
The distance between Anacortes and Snoqualmie Pass/Lake Easton SP is just long enough, and the spacing between known campsites awkward enough, that you're going to have one long tough 120km+ day, or you have to add an extra day and have four shorter, easier days. Tradeoffs...
Day Four could be extended to hit the Columbia and stay at Maryhill SP, but that makes it about 135km on roads with no shade or shelter from the elements and a fairly stiff climb up and over. Breaking it at Brooks Memorial SP makes Day Five longer but more of it is along the Columbia and on more sheltered roads. I haven't been to eastern/central Washington in years, I'd forgotten until I started looking at Google Maps Streetview just how barren that area can be!
Thanks for the Mountain Loop idea - if you want to put more information on that here, please do. Even if we wind up not using it, it could well be useful to someone else!