Dear Michael,
Thank you for your query.
One thing that is hopefully clear to you is that each document has its own element ids.
An element id is only valid within the document containing the element associated with it.
When you run a filtered element collector, you are running it in a specific document.
The results cannot be used in any other document.
At least not the element ids.
I assume that you can easily achieve what you need through the user interface.
Is that correct?
If so, then I would suggest that you do some research on what exactly is happening there.
While RevitLookup is a powerful tool to support that kind of research, it sounds to me as if your case might require some more powerful and intimate interaction, e.g. using the Revit Python or Ruby shells:
They provide a command line programming console and interactive programming REP loop (read-eval-print) that enable you to dynamically code, create variables, run filtered element collectors, analyse and search the results, etc.
I am pretty sure that the solution is simple and that you will rapidly discover it using tools like that.
I hope this helps.
Please let us know how you end up solving this.
Thank you!
Happy weekend!
Best regards,
Jeremy