How to query cached plans that contain a specific missing index? I see a potential missing index for a table but would like to first query all cached plans that have that specific missing index (so I can later test those queries after the new index is built). How do you do build such a query?  Can sp_blitzcache help here?
This isn't foolproof by any means but as a rough sanity check you can use the following query that leverages the DMVs to filter on the cached plan text's table name and index name that you plan to index:
SELECT 
    PlanText.[dbid] AS DatabaseId,
    DB_NAME(PlanText.[dbid]) AS DatabaseName,
    CachedPlans.plan_handle AS PlanHandle, 
    OBJECT_NAME(PlanText.objectid) AS ObjectName, 
    CachedPlans.objtype AS ObjectType, 
    CachedPlans.refcounts AS CachedPlanReferenceCount, 
    CachedPlans.usecounts AS CachedPlanUseCount, 
    PlanText.[text] AS QueryText
FROM sys.dm_exec_cached_plans AS CachedPlans
CROSS APPLY sys.dm_exec_sql_text(CachedPlans.plan_handle) AS PlanText
WHERE PlanText.[text] LIKE '%YourTableName%'
    AND PlanText.[text] LIKE '%YourColumnToBeIndexed%'

This leverages the DMVs sys.dm_exec_cached_plans and sys.dm_exec_sql_text.
Just replace the wildcard filters above with your table name and index. This will give you the actual query that the cached plan is for, so then you can re-test it after you create the index. I also included some potentially helpful columns like DatabaseName, CachedPlanReferenceCount (how many objects reference this cached plan), and CachedPlanUseCount (how many times this plan was used). So you could theoretically add an ORDER BY CachedPlanUseCount DESC clause to get the most popularly used queries too and prioritize your list.
One reason why this isn't perfect is if the query in the QueryText column contains a different column coincidentally with the same name as the one you're indexing, but from a different table, and therefor will be a false positive match.