You may want to talk to someone with more technical knowledge here, but I believe the reason you're warned not to hook a system to a projection screen TV is because of something called "Burn-in". Old computer monitors used to have this problem all the time. Basically, what it is, simply, is when there's a "static" (non moving) piece of an image on the screen (Let's go with the original Super Mario Bros. on the NES, the area at the top that says "Mario") - when that non-moving image is there for too long, it'll leave a permenant ghost-like impression on the screen. This is why Screen Savers for computers were invented oh-so-long ago... the constantly moving image told you the computer was on *and* did not allow something to get "burned" into the monitor.
Since most all video games are a) played for extended periods of time and b) have visual parts of the game that don't really change, like status indicators, it's just typically not recommeded to hook game systems to any kind of monitor (Television or otherwise) that's prone to getting an image burn-in.
If I'm correct in all of this (and I could be making all of this up off the top of my head...), there would not be a cable or such that would prevent this. Here's what I'd reccomend though.
Call your TV manufactur. See what they reccomend - if they say "NO!" then listen to them.
TVs have improved *a lot* over the last few years, so depending upon the age of your set, you *may* be okay to use it. It's likely that video game companies put that warning in there as a *just in case* disclaimer, so if someone does have an older set and it gets messed up, they can claim "Well, we told you not to do that."
Call Sony/Microsoft/Nintendo. Get their opinions. You'll likly get told not to do it (again, the *Just in Case* disclaimer), but maybe they can give you a better explanition as to why they say not to do it.
Don't play long games. One or two rounds of DDR might be okay, but if you're looking for a night long match up, just go with the smaller TV... Hook your system to more than one TV even...