I would suggest NOT disabling the relto book, for the simple reason that there's always the alt-f4 workaround for anyone who really wants to leave.
However, I love the foreboding atmosphere that this ... shall we call it... 'puzzle' idea created. The only issue is that the current age has a significant amount of 'ambient' light (not produced by an artificial source) look at all that purple light.
If it really was black outside the clearing, and the only light was produced by lights strung around the area, powered by a generator some distance from the actual puzzle... (some trivial tile puzzle sort of thing, perhaps, but the time constraint makes it more challenging) .. then, if the player lets the lights go out completely (while out in the open) it would get dark, you'd hear the sounds closing in, and automatically panic link.
(and for the initial case, the player would link into a closed, protective 'generator shack' of some sort, so that they could bootstrap the lights. (And you can hear the things clawing at the outside when you arrive... and the avatar would refuse to open the door until they went away. (note that we never actually see the things) ) ) .... yay nested parens.
(the idea really makes me think more of the stoneship age generator in the lighthouse/lights in the tunnels, rather than something as obtuse as pellets, but maybe I'm just less uru-centric in my myst-thinking.
)
Edit: This mechanic also lends itself to Uru-multiplayer, should it ever come to that. The situation could be made considerably easier/less tense by bringing along a second player to man the generator, much like having a second person standing on the plate in the Gahreesen generator-starting puzzle.