Lobelia Blue Moon is an old variety that takes the heat much better than the others.. there are also new cutting propagated varieties with better heat tolerance (techno heat and waterfall series).

I've been using the 'Moon' series of lobelia for several years and I'm really happy with it. I've tried other ones and they just didn't hold out like the 'Moon' ones. I do have mine in morning sun for about 4 to 5 hrs. I mulch and water deeply once a week. I'm re-reading your post, so I'm wondering, are you planning on putting these in pots or the ground? Mine are always in the ground. If you put them in pots, you can still mulch them. Don't let them dry out in the heat of summer or they could die back.
Mmsc, I'm going to look for the techno & waterfall series you mentioned. I'd like to give them a try.
Good luck Proudgma!

Kat