I would watch what you do with it. A spray-on sealant is not going to cut it; maybe if you brush on some powerful epoxy based stuff (and repaint it with it every year!) you could have the thing last.
Sorry to be brutal, but I've watched plaster pieces melt before. Now I do live in New Orleans which has insane rainfall; but plaster is not an outdoor material.
If you do decide to Thompson Water Seal it or polystyrene it, be SURE you do it all over. Several times. Water can build up from beneath just as sure as soak in from the top.
You could also put something on top that will protect it; say a glazed ceramic dish as a birdbath, or a metal birdbath or birdfeeder. If it extended over the footprint of the column it would add a few years to it.
I'd still double-seal it. With a brush on finish.