I'd say that the Wen Solo Plus is likely your best bet.  IT already comes with a third seat, and I've heard it said that it is better at solo than tandem - I've never paddled one

I have a Northstar, and have only solo'd it a couple of times kneeling at the rear thwart - it is my Quetico tandem - it was OK kneeling, but not all that comfortable.  I suppose you could replace the kneeling thwart with a seat, but I think it would be better placed a bit more towards center for sitting.   (I have 5 boats - that is the only tandem, so I'm prejudiced in favor of a solo rather than a tandem for soloing - that may color my opinion a bit.) 

There are a couple of other boats I can think of still available new that fit your bill of goods.  One is the Mad River Malecite- symetrical hull small tandem (often cited in the tandem/solo discussions) the new version in Kevlar light looks really pretty - similar in specs to the Northstar and Solo Plus - I think it used to be offered with a center seat option.

The Souris River Q 16 is another tandem offerd with a center seat options - i.e. you can get it set up as either  a tandem, or as the Q 16 solo version, or maybe with 3 seats. 

If you can find one on the used market, the old Dagger Reflection 16 was another boat touted for tandem/solo use and was offered wtih a factory center(3rd) seat.  Mad River makes the boat now, but its not the same boat.

If you are buying new, I'd see about special ordering a boat set up with a center seat and no bow or stern seats ( if you aren't going to use the boat tandem), or with a third seat option. 

If the thwarts are going to be a problem, you'll just have to reconfigure the boat for a center seat that works for you.  "Center" isn't accurate - most solo boats have the front edge of the seat at the balance point, so the seat is behind center.   For portaging, you would have to use a removeable portage yoke, or use the front of the seatframe in a Knu-Pack type of packframe rig, which is what I use on two of my solos.