Getting notification when log truncation is delayed
I'd approach this from a different tack and monitor for long running transactions.  You can query open transactions using the DMVs (sys.dm_tran_active_transactions should show it).  Here's a query (borrowed and modified from Gail Shaw) that will show you currently running transactions:

SELECT 
    ec.connect_time,
    tat.transaction_begin_time,
    tat.transaction_type,
    tat.transaction_state,
    es.host_name,
    es.program_name,
    tst.session_id,
    es.login_name,
    est.text
FROM sys.dm_tran_session_transactions tst 
    INNER JOIN sys.dm_tran_active_transactions tat ON tst.transaction_id = tat.transaction_id
    INNER JOIN sys.dm_exec_connections ec ON tst.session_id = ec.session_id
    INNER JOIN sys.dm_exec_sessions es ON tst.session_id = es.session_id
    CROSS APPLY sys.dm_exec_sql_text(ec.most_recent_sql_handle) est


From here, I'd set up a polling job against the DMVs and alert me if a transaction is held open for longer than an acceptable period of time.  What this amount of time is will be dependent on your environment.