We did the downeast loop last year. True enough about parts being pretty remote. But I'll say this: We stayed in marinas far more than I thought, and anchored out far less. Mostly because along the St. Lawrence and Gaspe, there are few anchorages.
Once you hit NS, the coast is pretty rugged from Canso to Halifax, but there are some fantastic destinations. Halifax is certainly worth a visit, and there are anchorages below that, although they get fewer and farther between as you approach Cape Sable. Once around the bend to Yarmouth, you have more options. You can cross the Bay of Fundy from there, or continue North and cross where it's narrower. Grand Manan Island is a good stopping point on the way over, if they've finally finished all the renovations at their three harbors.
Overall, it was a spectacular trip, and we're very glad we got to see all the things we did. Some of the more remote parts were monotonous after a while, and we missed out on Bras d'Or, but we'd gladly go back to most of the areas we covered.
You do need to be comfortable running outside and in fog, and waiting out foul weather when necessary. Details of our trip are at trawlercygnus.com
The problem with making an "even greater loop" by adding this to the Great Loop is that you'll be going against the current of the the Mississippi, Tenn/Tom or St. Lawrence.
Then again, the St. Lawrence does have reversing currents as far as Quebec, so with timing you could in theory skip the Hudson altogether and do the Down East portion first, head west through the Lakes and south on the western rivers.