I have a Jackson Elite seat that I strap to the pad eyes facing back, directly behind the Vantage seat. Your tiny wife would ride there like it was made for her. It is a VERY comfortable chair, not Vantage comfy, but not bad at all, but also not cheap either. It works great and back to back does not change the center of gravity much at all. Just about any low boy beach chair would work. You just have to make sure the feet distribute the weight over a large enough area that your not putting pressure on to small of a space.
If I didn't already have the Elite, I would have went with an easy to find substitute like the Larry Chair (cheaper) or the BassPro Ascend Deluxe Sit on Top kayak chair ($90). Both distribute the weight well, though the Ascend would be more comfortable for longer outings. They both sit low so as not to disturb the center of Gravity to much, but high enough to keep her bottom dry.
http://t.basspro.com/Ascend-Deluxe-Seat-for-FS12T-SitOnTop-Kayak/product/14081008121854/Not sure what Hobie has to say about this. I do know that the PA14 has a sticker clearly stating that it is a one person craft up to 600 pounds. Not sure what the water patrol might say. I don't use it that often, and if I ever got pulled over, which has happened a few times in my Hobie, I would plead ignorance tempered with regret. I have found most fish and game/park rangers and police are pretty decent. Each time I have been pulled over (so to speak) there have been the perfunctory questions: fishing license, whistle, etc. But mostly it is because they want to chit chat about the Hobie. Actually the last time they came aside it was to give me a tee shirt congratulating me on wearing my PFD.