PostgreSQL: Log statements which do sequential scans
I think the only way to do this is to use the auto_explain module and enable dumping of execution plans if a statement is slower than e.g. a second. 

The plan will be written into the Postgres log file

Then you can have a job that monitors the log file and takes actions if a Seq Scan is part of the plan. 



A Seq Scan is not something that should be avoided at all costs. It often the fastest way to get the result. 

I would focus on plans with Seq Scans, but simply try to find (and then fix) slow queries. 

For analysing the logfile you want to have a log at pgbadger or you can use POWA to monitor your server in real time