Such a thing is simply not possible to do in software on Apple Watch. The rules under which the screen activates are set firmly, and I can imagine could only be twisted if watch becomes jailbroken. Meanwhile, people came up with mechanical solutions to accomplish this, like this one:
Where an electric motor slowly spins the crown, which keeps the screen awake. Alternately, for a nightstand mode, a device could be fashioned that would create a small vibration every 10 seconds or so, and place it next to your watch. However, keep in mind that doing this would without question create a burn-in on the oled screen very quickly, most likely after just a few hours, and it could stay there permanently, or long enough that it looks permanent if you keep repeating this every night. This is a serious problem with OLED screens when displaying relatively bright, well defined image, like those green numbers are. I think overall you would regret doing this for how small the benefit is, and how large issue you would cause - for an always-on to make sense in the nightstand mode, they would have to program the clock numbers to shift slightly over time, which they don't do right now.
I am very curious how Samsung deals with this, as it has an always-on screen mode on Gear 3 watch. I've read that some people did notice burn-in after just a day of use, which went away when the disabled always-on mode. Others however said they did't notice anything.