As long as you can run arbitrary SQL, you can get all the information out. Assuming it can do something with it rather than just run a plain query. If so, I suggest you take a look at the check_postgres nagios plugin or the munin plugins for some examples of which SQL to run to get the interesting metrics back that you want. Some quick googling shows several examples of how to monitor with custom SQL queries, but I don't know enough (or anything) about SCOM to recommend any of them in particular.
Thanks, that is the conclusion I had come to as well. I have written a management pack for collection other Linux data so may have a go at a PostgreSQL one if I get some time.