First, depending on climate, it definetely should be brought in in winter.
second-use an epoxy grout. these grouts are designed for use underwater in pools and fountains,
3rd yes seal with a masonry/tile sealer- for the tiles themselves. The epoxy grouts need no sealer. it ought to work just fine.

So in my true, indecisive manner, after painting the birdbath I have decided to use glass instead of talavera (too thick/wrong colors). So the problem I have is that the bird bath bowl has a raised design on it (not much in the very middle but up the sides, flowers/leaves). If I want to even it out a bit before mosaic-ing, can I use some thinset to build up and fill in the gaps, let it dry and then mosaic? Or just dump a bunch of thinset in and put the pieces into it? I appreciate any help.

In the pic below, you can see leaves/branches/flowers dipping into the bowl. It is hard to see the ones I didn't paint but they are there. (ignore the tile, I was playing around with ideas)

Here is a link that might be useful:

I would smooth it out with some thinset, let it cure well-several days to a week, then mosaic on it. I like to work on a smoother surface rather than try to level that out as I go.