Hello @tjenner,
I am not aware of a tool like that.
If there was, it would probably be in a complete Revit application not in a simple tool.
A tool like you want needs information of Pipe Types and Duct Types in your project and based on that information it has to look up weight information somewhere.
So that is not an easy task.
Why don't you use schedules in your project and add weight information to it.
Add a project parameter 'weight per meter empty' and "weight per meter full" to the Pipe and Pipe Fitting category.
Per Pipe Type fill in the value for these parameters.
Then create a Pipe schedule like you see below:

You can do the same for Ducts.
For the Duct and Pipe Fittings, you could add the weight values in lookup tabels that are hosted in the families or create a Dynamo script that reads an Excel file and updates the weight based on the Fitting type and Size.
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Kind regards,
Robert Klempau
Senior Consultant AEC
Cadac Group AEC BV