I'm an AB in the SUP and I can tell you the SUP absolutely ships OS jobs. I just got off a ship and worked with three Ordinary's that had never been on a ship before. Here's the deal, the only jobs an Ordinary can get with the SUP are on MSC contracted vessels until they get the RFPNW. Then they become an STOS and can ship with APL. So your right you have to go to another company (in this case MSC contracted ship) to get your RFPNW before you can ship as an STOS BUT you are with SUP during that time.
The RFPNW hurdle is actually made easier by shipping with the union. You only need 60 days of watchstanding vs. 180 days so thats a smokin deal. You need three weeks of MSC schooling in San Diego to get on the MSC ships. The union paid for my school, hotel and food. They sent the Ordinary's to the RFPNW simulator class so they got their assesements signed off there, they just needed to stand the watches.
The reason they call is because there are never enough people with the MSC training in the halls. They do put the jobs on the board but since they can't fill the job this way they end up calling. This is because there aren't many of us and the jobs aren't on a regular schedule like the commercial ships another reason the MSC trained people aren't in the hall. All you do is let them know you are ready to ship and then follow up periodically.
As for MFOW your comments about the Electrician, QMED and Jr. are correct, they are in demand. It is tough to get a wiper job because there is only one wiper on an MSC contracted ship (at least the one I was on) compared to three Ordinary's.
In summary, if you want to ship with the SUP or MFOW entry level then the first step is to get them to send you to the MSC training, then keep in front of the port agent (either in person or on the phone.) It will be easier to ship as an Ordinary and probably a better career move long term to ship as a Wiper. IMHO...
Good Luck !!