Chapter 4 isn’t much help – it only tells you about using VAS for signs where the “SLOW DOWN” message is permitted. DfT don’t envisage using VAS for any other warning signs.
If you did use diag. 545 as a VAS it would have to be additional to a fixed diag. 545 sign (for the benefit of drivers travelling at an appropriate speed) and it would need a VAS representation of the appropriate plate under it (“School”, “Patrol”, “Playground”, etc.). I am not aware of any manufacturer making such a combination as a VAS and, as Andy says, the symbol is not one that lends itself to easy recognition if rendered as a pattern of dots.
So in summary, legal but strongly discouraged.