I think most everything mentioned in this thread would in theory do better in ground, I know that here for me they would, atleast what of it I'm familiar with, but when my ground gets really dry, there's no amount of water than can help my heavy feeders, it just wicks away into the dry ground around it and evaps into the dry air!
Atleast with potted plants you can keep bumping them up into bigger containers that have virgin soil in it, it'll hold water ok until the new roots colonize the dirt, you can also stick the pot in a few inches if water, generally a no no but when the plant drys it's pot out in 12 hours...it's not gunna hurt it much to spend a day or 2 a week with wet feet...
I too am tempted to thing the key to success here is get a year of good growth under their belts, then things will get easier, and planning ahead for big plants is essential!
I like the look of tropicals jammed in really tight!
It doesn't help the wimpy plants tho! And if a plants isn't growing at the same rate as his neighbors, it'll be quickly swallowed up by them...
As for the the potash, literally ash from an old fire is great, especially if worked in when planted, but just heaped around the base is fine... Even in drought stricken Texas I'd think you could find or generate some ash and charred wood...
Urine is a free source of nitrogen too, I rarely waste mine...some models even come with a handy dispenser!
