Yes you can fit a varistreamMK2 to an omnitrolley.
You have to cut the two wire's that go to the pressure switch but make sure there long enough to re-attach if the varistream unit ever packs up and then connect the pump to the varistream and disconnect the on - off switch on your trolley.
Williamsonpumps have got instructions on there site. It pretty easy to do
I have the varistream fitted on to my trolley and works a treat except for the flaming button which is getting rather stiff to turn on and off.
Paul