I know, that's crazy about the HHSS odds-ends sale price! Several people here have been waiting for that, big disappointment. I guess we have become spoiled, considering last time I picked up an extra I think they were $100 including the over cover! I guess there are never any guarantees when it comes to a sale. It must be the woodland camo thing, I didn't even know they had those.
But, it sounds like a good idea to just rig up your bag into an UQ if you guys have the sewing skills! You have to get it so that there are no drafts on the ends or sides, that will be the big factor. Plus, unless the bag has some kind of differential cut ( unlikely ) you will have to learn to adjust the tension so that there are no drafts, AND so that the UQ is just snugly contacting your back but NOT tight enough to significantly compress the loft. That is a new factor that you don't really have to deal with if the bag is just laying on top of you in normal use. But it is possible to pull an UQ tight enough to flatten the loft, which you obviously do not want.
But if you can get some shock cord sewn in place to accomplish that, your sleeping bag may work fine!
For I consider that the sufferings of this present time are not worthy to be compared with the glory that is to be revealed to us....that the creation itself also will be set free from its slavery to corruption into the freedom of the glory of the children of God. For we know that the whole creation groans and suffers the pains of childbirth together until now.