I work as a VLK at Watford and over the last 2 years, when the flight hasn't been locked overnight, I'm not aware of any significant issues arising 'out of hours'. One of the main tasks of the VLKs at both Watford and Foxton during the busy times is to manage the flow of boats up and down so that waiting is fair and kept at a minimum. This generally works well and, at the height of the summer, by the time my shift ends at 6.30 ish the queue at Watford, at least, is down to a couple of boats and that is manageable by those waiting. I guess time will tell if the same works at Foxton.
There are instructions on how to use the Watford staircase written on the balance beams and at the start of the flight but unsurprisingly not many boaters seem to notice that the instructions are there or take time to read them.
ETA: I've not heard that Foxton has/will be going 24hour either so have only seen the notice posted above.