It looks to me like ListAggregatedTestRunsForJob is the key function to update, called in pass_fail.go. This is sourced by the view TestRuns_Summary_Last200Runs in bigquery. Unfortunately I think that may actually boil back to a scheduled query populating TestRuns_Scheduled_Unified_TestRunsForLast200JobRuns. (but I'd be surprised if we needed a scheduled query to do this)
The data we need doesn't look too complicated.