As others have stated, the blanket will be no problem... just stick out appendages as necessary or shove to the foot of the hammock entirely. Really, not a problem.
Also, this blanket is not a revolutionary concept. There are many quilts that have footboxes with zippers or snaps, combined with drawstrings that let them open completely and lie flat in blanket mode.
You will find that bottom insulation is the far bigger challenge. 20°C (68°F) might sound like a warm temperature, but after you remain still for a while and body metabolism slows down, you will find your backside getting uncomfortably cold. Some sort of bottom insulation will be needed, and although closed-cell foam pads and air mats can work, they are in my opinion far inferior to a quality, well-adjusted underquilt.
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