Thanks for putting this together. I'm testing with this model.
It occurred to me that a simple solution may be to add a standard function in the custom command area that would allow you to trigger a prompt for a "transient" value based on a Business Rule. For example, you could define a Business Rule that was like:
Before updating a user (group, etc), prompt for value. The prompt could be in the form of a new window/form. The value could have property enforcements, etc. and could be passed along to the workflow, etc.