...just using an extra long rectangular bag instead of a $$$ UQ? Open up the foot end a little and slide the hammock right through.

This was just a thought I had before heading out for the evening so I don't have the time right now to look around and if I don't do something now, I'll forget about it.

What do you see as pros/cons of the idea?

If I understand what you are suggesting is to use a sleeping bag with the foot cut open as a hammock sock. I think it would be a little claustrophobic in there but it would keep your bottom side warm.

I think it could work if you could find a way to keep it suspended under the hammock.

I think it would work okay in colder temps, but there would be a space gap between you and the upper layer of the sleeping bag, so you'd still need to have something to cover and insulate your body, resulting in more bulk and weight to carry in your pack.
In warmer temps, however, I'm afraid it would not allow much air flow and would get quite hot and stuffy.
I like your inventive thinking though.

That is the idea behind the peapod. A lot of folks have done exactly what you describe. Take a look at some of the Shug vids; he does that. Also, look at 52 Nights Over the Ground (52NOG). djminnesota slept like that a few times.
The issue is that you can't lay diagonally. So you sag in the middle.

I bookmarked this page awhile back because I own this sleeping bag and thought I might give it a try with my hammock. Haven't tried it yet but the guy was pretty happy about using it as a hammock peapod-type quilt.

I'd consider the peapod if I knew it was going to be very cold, but in the summer in the BW it can be warm and cold at night, not to mention buggy, so the peapod may have drawbacks.

quote luft: "I bookmarked this page awhile back "

Oh, yeah, he wrote the book on hammocking, literally.