Hi Ray, did you progress on this at all?
Couple of thoughts on this.
I don't think that cooking the onion any longer will make it any sweeter than it would be after cooking it as long as you have. I would suggest that the sweetness comes from other ingredients.
When I did my first base I added a portion of fresh lemon (including pith & rind) to it on the back of comments made by the IG Head Chef. This actually gave a fresh sweetness to the base.
Following my recent ABC tests, I felt underlying the foundation to all the dishes was aromatic and sweet. This in my opinion, is due to the the use of Cassia Bark.
Perhaps an experiment of using these two ingredients in small quantities, will help provide you with the missing sweetness you seek?