I don't know about plums, but I don't see why it would be different (thinking back to my plum trees it probably would've done them good to be trained like that).
I do it on apple trees. I tie them down. Pull them down and stuff them below another branch. Tie them to other branches. Stuff sticks between them and something (like the trunk) to push them down.
Basically anything you can do to hold them in the position you want them.

They are like any other branch other than sprouting in the wrong place and growing vertical.

I would say its worth a try if the water sprout are new growth, By new growth I don't necessarily mean this years but young enough to bend and be trained easily.

I would not try it if they are already well established though because if could crack where it joins to a main scaffold and let pests or disease in.

I asked the same question here a few years back. The answer that came back is "Yes". And, since then, I've done so quite a few times. If you catch 'em early, you can use string/weights/clothspins, etc. to train them any way you want. Have fun.
-Glenn

Just wondering how you went Klew now that 12 years have passed or even anyone else who tried to produce good fruit from water shoots

Yes, I changed the architecture of 5 acres of persimmon trees, bending and tying water sprouts and eliminating horizontal scaffold branches, over a period of years. Bending the sprouts far enough to point straight down allows the tree to have a compact size. Since the bent sprouts form an arch, it's a very strong structure, requiring no support. Fruit is borne at picking height and kept off the ground. Once tied, the branch doesn't shift from uneven fruit loads, avoiding sunburn and pancaking of one branch onto another.

Question? Why doesn't a water sprout produce fruit? I've heard that the branch needs to be near horizontal to produce fruit; but why??

HU..1753 - I've also bent water sprouts over. I've tied a lot to the deer fencing. But I wonder why the angle of the branch should matter. I get fruit on the leader of my spur dwarf Rome apple and it's vertical. If water sprouts will fruit then it's not the fact that they're water sprouts that keep them from fruiting??

Admittedly, I don't have access to the reasoning around branch angles and fruiting. As in your case, I first noted fruiting on a vertical water sprout that hadn't been removed the year before. I later found water sprouts that had bent over, under the weight of their fruit, forming an arch, and pointing at the ground.