I think the issue with keeping hot ends hot when idle is always going to be a difficult one to crack - even more so for water soluble support materials like PVA. You going to get some 'caking' keeping them hot even if you do a full retract - there's no obvious solution except a hot end that can heat up and cool down an order of magnitude quicker than current models.
I am fortunate enough to run a brand new Stratasys F370 at work. The hotends heat mind blowingly fast. They will heat from 20C to 250C in less than 50 seconds. It is crazy when they first start, they will rise almost 10C a second until 150C. I thought my 40W cartridges were fast.
What we need is a slicer that can anticipate this so it will begin to heat a nozzle before it is a ready for the tool change.