I too would like such a functionality.
However, in the meantime you can create a globals table with say Input1, Input2, etc and use them as variables over and over whenever needed( like we did in the days prior to $ & $$ variables). Just be sure to clean up after yourself.
No plugins needed unless you would like more than three inputs.
I have found plug-in use to be very helpful in some situations. Don’t always shy away from plug-ins, in my case they have become a friend to my development efforts.
Yeah, I do use 'globals tables' for this kind of thing, but it would just be really nice if the options for that Custom Dialog script command were spruced up a bit :-)
Plugins are fair enough, but having some of the features they offer 'baked in' to FMP itself is always a bonus...