A value list with output options is hooked to a length parameter, also with two values. You can edit the length with hotspots, so you can graphically switch between the output choices.
Here’s a simplified drawing symbol object which shows how it should work. Graphical editing works mirrored and unmirrored. (Though I am getting a strange error in the parameter script, it says value list values are duplicated ‘0’, but they aren’t. Also the parameter script doesn’t fail. See comments.)
Also, here is a label version of the same thing, all I did was change the subtype. The graphical editing works when unmirrored, but fails when mirrored.
Thanks for your help. In the label it is ‘nice to have’, but if it doesn’t work I won’t pursue it.