You would generally purchase a detachable yoke to go with your solo boat.  It is a bit of a pain to have to attach and detach it for every portage and remembering to bring it along with you, but it certainly works better than the alternatives.

I've always got a removable yoke when I solo. It's not REALLY something else to carry around cuz you're using it to the carry the boat when portaging and otherwise it sits in the bottom of the boat.

One thing to consider, if you have a backpacking style pack that comes up high over your shoulders, it can be hard to single carry. The yoke doesn't leave much space between the seat to fit the top of the pack in. You can move the yoke further forward but then it doesn't balance well. 

Thanks so much for the replies everybody.  Much appreciated.  Eggspecially you, PaPaddler....

Looks like Sanborn canoe bought the molds. The Merrimack Baboosic is now the Sanborn Bradley Nyborg. 

At the risk of boring everybody, I looked into the Sanborn/Merrimack thing a bit, and wrote to them with a few questions.  So, in case anybody is interested: 

Merrimack remains an entity that makes the standard line of Merrimack canoes, to include the Baboosic, in their traditional colors (red/maroon, green, blue, off white - I think all four look great).  They also make the Sanborn line of canoes, which I think means they are using the same Merrimack molds for the Sanborn boats, maybe there are one or two different molds, I'm not sure.  Sanborn boats have a few custom features like cane seats rather than the traditional Merrimack seats, and it looks like they have a whole different paint scheme.  Their version of the Baboosic is indeed the Bradley Nyborg by the way.   The Merrimack also has rings on the bow and stern.  I don't know if that is a vintage feature that has been restored or something entirely new.  My 1993 Merrimack does not have them.  I have mixed feelings about that.  I don't want to hear them bonking around, and I worry that using them to tie down on the car could wear the Kevlar a bit, but at the same time it would be undeniably handy.  Lastly, and most important to the original purpose of this post, they are working on a couple of designs for detachable yokes for the Baboosic.