Sometimes an application, such as caffeine-indicator, which allows the user manually to inhibit the screensaver, does not have a window to use with xdg-screensaver suspend.
In such a case, or even in others, it would be convenient to use a process as the "locking entity".
xdg-screensaver suspend # note: no window ID
should start a process that suspends the screensaver, and resumes it on SIGHUP, SIGINT, or anything else it can catch.
Indeed, for most modern desktops, even kill -9'ing the process will not leave the screensaver inhibited, as the "resume" action is a no-op.
While nice in some circumstances, I don't think this suggestion will ever get implemented, sorry.