The tub has a gasket, and no drain You'd make a mess every time imo.
I think you're too worried about burning an element. It takes a few minutes and usually smells real bad as the wick burns. Even burnt, they aren't very pricey.
A few drops to prime the top wick and you should have smoke.
If it never starts smoking, don't add more.
First check for heat /power etc, then add just a few more drops (your nose can usually sense the heat if your fingers cant) I never add a full dropper unless I know I'm going fully dry and it's warm, and going to run a long time too.
Some brands do evaporate "fast" while at rest. I think Lionel's always evap. less at room temp than others. With Lionel's; I've had trains start smoking well right away, years after being boxed with the last fill up, before I even thought to refill them again.