One trick I use is to make up a very very sloppy small feed, i.e. pretty much like soup, with a handful or two of roughage (Hi Fi Molasses Free and Readigrass) and some soaked Speedibeet. I add a few Spillers Condition Cubes on top if it’s a really fussy horse. As long as it is really soupy, they will get some water down them, which is better than nothing!
If you suspect that your horse might have ulcers, this is a good idea anyway, as a small roughage feed about 20-30 mins before exercise will prevent splashes onto the top half of the stomach lining, i.e. the bit that is subject to ulcers.
The only thing is to make sure you stand well clear as they slurp, as otherwise you’ll get sprayed with brown lumpy water as they do their best bilge pump impression!