As a train dispatcher at Watford Junction I'll clear this up:
Firstly the dispatch of Virgin Trains at Watford is done by London Midland staff, there are no Virgin staff at all at Watford.
Services towards London Euston are set down only, and if we see somebody attempt to get on the train near to us we will stop them, however if somebody at the opposite end of the platform attempts it we'll let it go, and the train manager will take it from there after departure (usually a full price single from Coventry).
Services away from London are pick up only so thats Eustons responsibility to stop people boarding, we won't stop you alighting of course. The risk being you get ticket checked before Watford and then charged a full single to Coventry.
There is one chap who regularly jumps on the 21:15 set down service, and is always chucked off without fail.
There is a LM service to Euston at the same time as the Virgin Trains so really there is no excuse to be boarding a set down only service.
The platform displays also state:
1st DO NOT BOARD
This train calls at this station to allow passengers to alight only. Tickets from this station are not valid on this train.
It might make us sound like complete jobsworths but if Virgin catch on to LM staff allowing passengers onto set down services, we'll very swiftly be told to be on our way.... to the job centre. And of course none of us want that, so these rules are enforced.