...this plant in heavy clay, or...? ... where there's an occasional water level of 10' below ground surface (after some occasional, unnusual, heavy rains)?

We have both circumstances,
yet we'd really like to have a Mexican Bush Sage,
if possible!

Any knowledgeable / experienced suggestions?
(As I suspect it is a problem, since they 'like' good drainage.)
What about adding sand?
Wouldn't that help the drainage better than only mulch?

Our old house had very heavy clay (couldn't even dig one shovel depth down because it was so compact) and we had trouble with Salvia and Lavandula. The soil eventually did break down after tons and tons of compost but in the end, we mounded the garden up and that was much better for us and the plants.

So, I would suggest adding mulch to your garden twice a year. Mound up the areas where you can, especially where the Salvia will be. Hope this helps.