I'm not clear on your use case.
But you can set arbitrary data values (not bound to fields) that are available in the data object in other scripts. E.g.
data.$myValidationMessage = "...";
My use case was making a call to a DDS and returning a validation message depending on if the request was successful or not. I didn't want to put this in the validation script for because I cannot control how frequently the validation script and therefore the DDS would be called (I don't want to make repeated DDS requests). I would rather make one DDS call in my Change script and set the validation message there.
Ok, I get it now. Using the data.$customVariable method I described above should satisfy that use case. Please let me know if it does. Otherwise I can provide some more advanced examples of how we've resolved similar use cases in the Avoka Exchange program.