The idea is to add a new optional configuration property to the BigQuery data source.
The user could then fill it with a path to a file stored on GCS containing the UDF functions ( gs://bucket/udfs.js ).
If this configuration is present we would add it to the BigQuery job definition and the user would be able to use the UDFs in his/her queries.
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For anyone stumbling at this discussion in the future: @fabito made a pull request that implements this and it will be part of the 0.12 release.