Ask yourself this: Is it really worth it to go to the trouble of theft-proofing your sliders? Who steals sliders? I would guess that no thief is going to take the time to undo a set of normal bolt-on sliders. For that investment in time, they might as well break in and steal the whole truck.
This just seems like one of those projects that could have the money and time it would take to accomplish diverted into something more worthwhile. Buy another toy for your truck with the money. <img src="/forums/images/graemlins/smile.gif" alt="" />

<Edit> Oh yeah, I'd vote for just welding those bad-boys on permanently. It provides that anti-theft function and stiffens the whole assembly. In the 5 years I've had my sliders welded on I've never come close to wanting to take them off for any maintenance or what-not.