This function was added to Totem in 2003-ish but subsequently removed at some point owing to a percieved "lack of maintenance" of XScreenSaver.
See below a simple plugin which will call xscreensaver-command -deactivate >&- 2>&- & once per minute while Totem is both playing and fullscreened. This effectively prevents XScreenSaver from blanking while a video is playing in fullscreen.
The code is very simple and could be easily adapted as a generic "watchdog" plugin for Totem to run an arbitrary command at arbitrary intervals under configurable conditions. I have tested it a bit but not a lot. Bug reports are welcomed.
To install, either drop the two files below in to ~/.local/share/totem/plugins/ to install the plugin for the current user, or in to /usr/lib/totem/plugins/ to install the plugin systemwide.