last week I finished pspg 3.0 https://github.com/okbob/pspg . pspg now supports pipes, named pipes very well. Today the pspg can be used as pager for output of \watch command. Sure, psql needs attached patch.
I propose new psql environment variable PSQL_WATCH_PAGER. When this variable is not empty, then \watch command starts specified pager, and redirect output to related pipe. When pipe is closed - by pager, then \watch cycle is leaved.
If you want to test proposed feature, you need a pspg with cb4114f98318344d162a84b895a3b7f8badec241 commit.