i think it's a great idea. you can spend a couple of days en route to the san juans starting in downtown Seattle.

hop a ferry to Bainbridge island, ride to and spend a day in Port Townsend (one of the cooler waterfront towns on the west coast), ferry to Whidbey island, ride to the san juan ferry terminal in Anacortes, ferry to San Juans.

I lived in seattle until fairly recently, and can't think of either a touring bike rental or a tour operator out of Seattle......you're pretty much going to be on your own and buying panniers, i strongly doubt you'll going to be able to rent them.

maybe REI, Seattle's flagship store and the headquarters, might, just might be able to help out.