looking for missing indexes in the cached execution plans I have the following query that shows me what is cached in sql server:

SELECT cp.objtype AS ObjectType,
OBJECT_NAME(st.objectid,st.dbid) AS ObjectName,
cp.usecounts AS ExecutionCount,
st.TEXT AS QueryText,
qp.query_plan AS QueryPlan
FROM sys.dm_exec_cached_plans AS cp
CROSS APPLY sys.dm_exec_query_plan(cp.plan_handle) AS qp
CROSS APPLY sys.dm_exec_sql_text(cp.plan_handle) AS st
WHERE 1=1
  --AND OBJECT_NAME(st.objectid,st.dbid) = 'YourObjectName'
  AND query_plan IS NOT NULL
ORDER BY ExecutionCount DESC




Is there a way I could query the queryPlan field looking for missing indexes?
You can try to execute below script and find for missing indexes in cached execution plans from here

SELECT qp.query_plan
, total_worker_time/execution_count AS AvgCPU 
, total_elapsed_time/execution_count AS AvgDuration 
, (total_logical_reads+total_physical_reads)/execution_count AS AvgReads 
, execution_count 
, SUBSTRING(st.TEXT, (qs.statement_start_offset/2)+1 , ((CASE qs.statement_end_offset WHEN -1 THEN datalength(st.TEXT) ELSE qs.statement_end_offset END - qs.statement_start_offset)/2) + 1) AS txt 
, qp.query_plan.value('declare default element namespace "http://schemas.microsoft.com/sqlserver/2004/07/showplan"; (/ShowPlanXML/BatchSequence/Batch/Statements/StmtSimple/QueryPlan/MissingIndexes/MissingIndexGroup/@Impact)[1]' , 'decimal(18,4)') * execution_count AS TotalImpact
, qp.query_plan.value('declare default element namespace "http://schemas.microsoft.com/sqlserver/2004/07/showplan"; (/ShowPlanXML/BatchSequence/Batch/Statements/StmtSimple/QueryPlan/MissingIndexes/MissingIndexGroup/MissingIndex/@Database)[1]' , 'varchar(100)') AS [DATABASE]
, qp.query_plan.value('declare default element namespace "http://schemas.microsoft.com/sqlserver/2004/07/showplan"; (/ShowPlanXML/BatchSequence/Batch/Statements/StmtSimple/QueryPlan/MissingIndexes/MissingIndexGroup/MissingIndex/@Table)[1]' , 'varchar(100)') AS [TABLE]
FROM sys.dm_exec_query_stats qs
cross apply sys.dm_exec_sql_text(sql_handle) st
cross apply sys.dm_exec_query_plan(plan_handle) qp
WHERE qp.query_plan.exist('declare default element namespace "http://schemas.microsoft.com/sqlserver/2004/07/showplan";/ShowPlanXML/BatchSequence/Batch/Statements/StmtSimple/QueryPlan/MissingIndexes/MissingIndexGroup/MissingIndex[@Database!="m"]') = 1
ORDER BY TotalImpact DESC


It has sort of worked for me in sql server 2008 and 2012, but still you will need to check if that works the way you looking in sql server 2014.

Also,

In addition to above i would suggest you read Finding what queries in the plan cache use a specific index , a great script from Jonathan, which really helps in understanding the usage of those indexes getting used for the current execution plans in cache.