Make FloatProperty library overridable

This is a follow up of sorts to this thread: Looking for Python programmer to update MakeHuman MHX2 addon (paid) - #8 by Unluccy

I had a programmer (which I am not) update an old rigging addon, which works fine now. However, I noticed that the FK/IK switch variable is no more library overridable. Not knowing a lot about Python or the Blender API I tried to fix it by googling and arrived at this:

bpy.types.Object.MhaArmIk_L = FloatProperty(default=0.0, min=0.0, max=1.0, override = {"LIBRARY_OVERRIDABLE"})

However, not only does this not work, I even noticed that the “min” and “max” settings (which I didn’t touch) don’t work either.

I’m pretty sure there is just some mistake in it or that this needs to be set in a different way but I cannot figure it out. Does anyone know how to do this right?