Wayne,
Please try ROCS Data Extract.
https://bitbucket.org/reltio-ondemand/util-dataextract/src/master/
This can be easily adjusted to meet your requirements in case the results don't meet the requirements.
Another option could be to add a nested attribute with Source System Name and Source System ID in L3. This would be overkill. But this will allow you to search against all of the Xwalk IDs and values in the UI.
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Radhakrishnan Ramalingam
Architect
San Ramon CA
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