Yes - you'll find it much easier if you loosen the cable at the handlebar end and unhook the barrel that is crimped on the end of the cable from the handlebar it will give you some slack to work with - though you may find it is easier to also unhook the end from the carbs - it is a PITA, but you basically have the piston inside that is pulled snugly from the carbs by the springs, and held fast by the handlebar end. Unscrewing the threaded nut on the end will give you a bit of slack, but it may not be enough to fully get everything apart.
The plastic itself doesn't need lubrication, but if there is dirt collecting in there, it will bind up the piston from sliding easily. I clean things out in there, but just use a little grease on the cable ends and nowhere else.
These bikes had a small plastic or rubber flap that would hang down under the tank and block some of the water, etc. from getting on the splitter and other stuff under the tank. But it isn't always effective. You can tape up the ends (especially the front end) to help seal things, but don't use anything that will prevent you from unscrewing the nut and the front cover to get it apart. The main thing with tubing the inside (in my opinion) is to prevent corrosion of the steel cable ends and the metal plugs crimped onto the ends of them - once that stuff starts rusting, it causes alot more friction and prevents the cable from rotating which helps it slide in the cable sheath.
