I think the ability to pass a custom parameter in a URL would be extremely valuable and not hard to implement. It would allow us to avoid having to maintain multiple copies of the same form with the only difference being JavaScript parameters. Presently, adding anything to a UCI URL causes an error.

As an example, some of our users would like to open multiple windows with the same form, but have it open in a related item (associated list view) for Orders in one window and for Contacts in another window, so that they can refer to the main form while working on Orders and Contacts. They currently have to duplicate the form, go to the Related menu, scan down to the related entity view, and click it. I want to automate this with a single button on the Account form ribbon that runs JavaScript to open the two new windows and then have each of those windows open the associated list view for the respective related records. My wish is that I could add a parameter the URLs for these two new windows so that the JS on the form would know what to do by parsing the URL for this parameter. My alternative will be to have the JS store the parameters in local cache and then clear it so that it doesn't affect future windows.

Note: There may be a sneaky way to pass data by embedding an encoded comment in the recordSetQueryKey parameter.
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