I still have to fix some details (presets and shadow) then I'll upload a futon, but since
I'm a lousy modder I'll settle for a simplified version because I lack the skill and the
patience to solve some issues. But if someone will ever want to improve my mod or
make another one, I'd like to write here some notes that maybe can be of some help.
I copy/pasted the sleeping bag script to create a custom one in order to remove the
"Put in inventory" interaction, use a custom jazz script (so I can use my own version
of some animations) and make other minor changes (remove the debuff you sometimes
get, like the backache, for example). I made copies of the sims's animations to get in the
bed and leave the bed in order to remove the feathers special effects. I made copies
of the sleepingBag animations cause I needed to make some changes there too and
this is were I failed.
In case you are wondering about legal issues, even ignoring the [minor] changes
I made, all these resources make part of a patched basegame, they were not
been taken from the Generations EP, so they are redistributable as a mod (right?).
In the last post I said that I got a warning from MilkShape when exporting the "closed"
(I'm wrongly using this word as opposed to "rolled") sleeping bag modified mesh
(it seems that if you move a vertex it loses its "link" with the joint it was assigned to),
well, that's part of the problem too.
What I mean with "simplified version" is that I eliminated the object animations and since
it's difficult for me to clearly explain the reason with words I'll try to show you what I mean
graphically. Here you can see some frames from two animations that run simultaneously
and perfectly synchronized, on the left there's my prototype and on the right the regular
sleeping bag.
[the tag-lines refer to the image below them]
Routing to the object completed, the sitting animation is coming and will be followed
by the "get in the bed" animation.

Feathers special effect removed, even when sims wake up (had to "clone" the CLIPs,
fortunately those resources are relatively small sized, and use S3PE's "Grid" to delete
that specific special effect entry). As you can guess looking at the shadow, the "sheet"
is bending (and I don't want that), but I'll explain that better with the next image.

This is when sims wake up, but happens also when they go to sleep as mentioned.
As you can see, the bed sheet (not sure if it's the right word, I mean the part of the
bed under the sim) bends like it does for the sleeping bag, but that looks awful since
we're trying to do a futon or floor level bed.
He didn't have a nice dream, that "angular monstrosity" is probably due to the warning
I mentioned earlier about an unassigned joint.

This is the first frame (as showed in MilkShape left viewport) of the CLIP
a2o_sleepingBag_stop_sleepingBag. To avoid the "blanket's bending" I though
about two solutons: edit the animation leaving in all frames the joint at its initial
position or unAssign the joint in the sleepingBag mesh. None of them worked,
I also tried some variations (for example keeping at the same position other
joints, like _bind_1_top that seems a "father" joint).
I probably made some mistakes and maybe stopped trying too soon (after modifying
the animation I saw some changes but after several attempts without good results I
got discouraged - keep in mind that meshing and animations are not my thing) and
I decided to remove the animations completely, so in my mod you'll see a sudden
change of the futon mesh instead of a more smooth transition granted by an animation.
Another reason why I give up on the object's animations is that I'm now using a new
mesh, completely different, and I don't have the skill (and, again, the patience) to
correctly assign the right vertices to their respective joints.