Scripting the lock-screen function in xscreensaver?

[Jesse Sheidlower]

I'm running xscreensaver as part of Gnome, and would like to be able
to control it in a way I can't figure out.

Most panel icons can right-click to a "properties" window that gives,
among other things, the command that will be executed by clicking on
that icon. The "lock" icon does not have this; the properties window
just controls the aspects of the display, the timing of the blanking,
etc.

I'd like to change the action of this icon so that, before locking
the screen, it executes (for example) "ssh-add -D" to clear all of
my ssh-agent's identities.

Similarly, I'd like to know the command for locking so that I can put
it elsewhere; for example, into an rc.suspend file, so that if I
close my laptop's cover it will automatically lock the screen.

This at least I would have expected to find in the man page, but 
perhaps I missed it.

Thanks.

Jesse Sheidlower