Huh, I cannot find a decent pic.
OK, so the adjustable seat post on a bike is a pretty complex geometry.
Wide at the base, in contact with a bike frame.
Narrow for a stretch at the top portion.
Knurled inwards at the very top.
To insert a backpack/rucksack mount would require 3 widths then.
It need to be wider at the base to not slip out.
The mount would still want to pivot sideways.
I'd almost consider tacking a square tube onto the base.
But this would then require a different seat-rear-mount part.
It gets complicated in a hurry.
Also, I don't think the pipe will clear the seat back. I'd hafta chop a notch out of the top of the seat back.
Then there is the issue of clipping the back of the rider's head.
It might make sense to
1) make a headrest attachment,
2) recess the pack mount rearward with a zigzag in the tube.
Then we still need to address a telescoping tube to adjust for a rucksack!
A 2 part fold-out arm might be better.
1) the first part on each side swings up, and the two halves clip together.
2) and additional hinge point then open to simulate shoulders to hood a backpack or rucksack on to.
Rucksacks are packed for hiking. That means the heavy stuff at the bottom.
They will not present much air resistance if mostly behind the rider.
This is unlike panniers on the front and rear tires, for example.
Also, I like a human not bike centric storage system.
I have NEVER liked panniers for this reason. They are not easy to carry.
I want a bike I can use to get to a nice location -and then I go hiking.
That means I take my storage with me. Or it gets stolen.
That means ... a backpack. Or a rucksack.
A recumbant seat back precludes simply wearing a backpack.
I suspect a seat design could be built that would attach the rucksack directly.
This might ultimately be the superior layout.
It is far to invasive for me to attempt now though.
I'd just ruin my seat!
I think I need to drill a hole thru my seat tube. Use a pin or bolt to keep it from rotating.
Accept that my prototype will not telescope or have all those hinges.
It just needs 2 settings:
1) short backpack, completely behind the seat,
2) tall rucksack, behind the rider's head.
Obviously this layout allow a decent partial fairing.
Which is exactly what I had in mind!