I baked the headlights on my Supra in '98 to get the permanent haze out of them. I was the first Supra owner to do this to my knowledge. I posted about it on the MkIV list, and ever since, I've heard of guys baking their headlights to split them apart. (Then they stick them back together with silicone, how will they EVER come apart again if they need to????).
The parts department sells the black string Toyco uses to seal the lights. It's less than a buck. They sell it for taillight installs because the taillight needs to be sealed and the wires have to go inside. It's cheap and easy to apply, and it comes apart EASILY with about 150 degrees of heat (I wouldn't go up to 250, I'd be too nervous about melting it).