I can't find a current reference or photo, but we used to have one at my previous shop. If I remember correctly it was manufactured by J.A. Stein, but that may be wrong. It was a professionally made copy of a popular homemade wheel building tool.
It was called a Stress Relief Box. It consisted of a 6 inch deep box, about 3 inches wider than a 700c rim. There was a a hole cut just about 2" smaller than the rim in the top of the box, so that when a wheel was set on top of it, the rim was supported, but the spokes and hub were not touching anything. There were inserts to allow use with smaller wheels as well.
There was an metal arm that straddled the wheel, which looked like a taller, and wider version of the Park tool dishing tool, with a lever that allowed pressure to be safely placed on the center of the hub.
In this version, there were no hydraulics, but it would appear similar to a hydraulic press, or there could be more versions of them. There were lots of homemade ones, so there is also a possibility that it was a DIY version. I know a guy in Seattle who was working on a Arduino controlled version which was supposed to allow you to set a tension level.
The one we had worked well, and required minimal setup, although it took up space. I haven't seen one in any shop since, and I can't find a current reference, so it may be off the market.
This a very rough diagram of what it looked like: