We have a field we use as an identifier for customer transactions.  When this field is populated by a technician, I want to have that value checked against all other requests to see if it has been entered before and popup a message to the technician.

I know how to create an alert to create the popup, but the API doesn't provide a means to search through all requests for a user defined field, unless the documentation is out of date or i'm just dumb.  I have a view i've created that i can query the database against for the submitted value via an external script, but other than using the API to update a value in the request, i don't know how to make this popup a message in the request itself.  

Can the write custom script tool on the field and form rules be used to run an external script and accept data from it?  or is there another option i'm just not seeing how to do this?