Mounting bushings aren't shrinky-dinks. They're only roughly 10mm in diameter to start with; freezing them isn't going to change their size enough to measure. Wax probably isn't a bad idea; I wouldn't use vaseline. It never dries and will probably soften the wood.
If you want to hammer them in, I'd use something larger than a dowel to start with. A hunk of 1" x 2" or larger wood. Less chance of missing it and hammering your top. Be very careful about the bushing being square with the top. It's easy to get them started on an angle, and once that happens the hole gets distorted. Once they're down close to flush with the top, then maybe a dowel to drive them the last bit.