Okay, so... The experiment with Envirotex Lite was a qualified success. It took around four hours to cure in a rather cold man cave, and there were issues with the masking tape used to seal the ends. Even so, the result isn't too bad.
I put a drop each of yellow and blue acrylic ink into the mix whilst stirring. It did look too green at first, but I think it's about the right hue now, especially with the colours of the green or beige cloth underneath. About an hour before it cured altogether I managed to create a few streaks in the varnish to give the impression of flowing water. The darker green patches are where the masking tape acted more like a full-on dam than a restraint. I think using a little less varnish in the next pours should do the trick.
I'm going to experiment with Pledge varnish with green ink next to see if that will give a comparable result. If so, I'll probably go with that method since it'll be easier, cheaper and far less messy.