On the two occasions I have been SSSS'd for a Virgin flight, the VS website has allowed me to check-in & confirm seat selection, but it stopped there and would not issue a boarding pass.
Personally I think printing the SSSS on a boarding pass defeats the purpose of the supposedly random selection for SSSS. I am sure the SSSS is embedded in the boarding pass barcode as when they scan your boarding pass, the light goes red if you have SSSS. By printing this on the pass, you are alerting anyone who is up to no good that they are in for a secondary search at the gate - so if they have stuff on them that they shouldn't have, they'll either dump it or just walk out of the airport and miss the flight.
I know the G4S screener at the gate is not exactly law enforcement - but the Police don't phone up criminals and give then advance notice of a house-raid (for good reason too, as they want to catch them in the act).