The PRV isn't flush with the lid inside so a little will be left. On another thread I saw the suggestion of trimming the gas-in dip tube flush with the inside of the keg. Then tip the upside-down and pre-pressurized keg a little so that's the lowest point and let the sanitizer settle. Then blow it all out by popping a QD on the gas post. If you want to take all the trouble, that sounds like the emptiest you'll ever get the keg.
But there may still be foam, and I don't like to use Star San as a no rinse except for spraying or dipping little things where it's the easiest option and not much will be left. Never on a fermentor or keg, there's just too much chance of a nasty taste. And have you noticed the smell/taste/sliminess of Star San gets worse the older it gets? Where possible I use no-rinse (12.5ppm) iodophor. Drains well. (I just clear it out from the liquid side and have no taste issues, but i dont have trimmed dip tubes, so probably less than an ounce is left.)