Adjust soft limits from the UI?

Geosec wrote:

The way I do it now is to have two different setups of LinuxCNC... one for with the vice and one without, and launch the appropriate one on startup.

That is exactly what i wanted to suggest.

You can access the limit values through hal like this example:
net limit-x-max-set <= your_x_max_limit_value => ini.x.max_limit
net limit-x-min-set <= your_x_min_limit_value => ini.x.min_limit

so you could switch between two fixed values set in the ini file or i suppose you could also use a pyvcp slider to set it.

What UI are you using?

Gmoccapy offers custom panel integration using Glade. Installing the Glade editor can be a bit of an adventure, at least the last time I did it some years ago.
So you could have a toggle button in the gui to toggle between two sets of limit values defined in the ini or you could also add some spinboxes to enter the limit values in the gmoccapy gui itself.
If you just want to change the limits outside of gcode execution then I think that is all that is needed if you want to switch them while gcode is executing you need to add a queue buster to get the motion planner to work with the updated limits. Some experimenting may be required.
As for how easy it is to implement, that would depend on your skills and/or motivation of course.