The first problem to overcome with moving the offset manually is that the Extend side of the cylinder requires a larger volume of fluid than the Retract side (because of the piston rod) so connecting the two together would require some type of reservoir for the extra fluid when not fully extended.
Given that you state you would not adjust with any real frequency (and the assumption that you're really only looking for full offset, full retract and maybe?? a couple of positions in between & not infinity variable) - I would suggest removing the hydraulic cylinder and replace it with a collapsing pin-able linkage set-up.
Traditional screw links would be too slow & cumbersome to adjust esp for full travel, a screw jack like a trailer tongue jack would also work but again I think would be too slow.
It shouldn't be too hard to fabricate something from either round or square tubing and some flat plate to make your attachment brackets. Google Telescoping Tubing.
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I have also been thinking about a "Ditch/Bank Flail Mower" that has both Hydraulic Tilt & Offset. It's going to cost me about $800 to add 2 factory rear remotes and this would be the only thing (except maybe a Top & Tilt) that I would ever use them for, so I have thought about just running four hoses off the loader control QD's to the rear for the mower. Maybe that's an option for you - run the offset from the Curl/Dump function of your loader control.
[Edit to add - This is what happens when I try to post from work and keep getting pulled away. As you can see me & LD1 think alike - he should probably be scared. ]
