Anyways, couldn't it be argued that a piston is the best filler?

No.

For efficiency...either you go with what Frank described with the sheaffer
plunger (single stroke, massive flushing action, fully visuated chamber, 2 to 3
times even the most efficient telescoping piston capacity per barrel
size.....example...a sheaffer tuckaway holds more ink than a pelikan 1000 yet
it is the shortest/smallest of all plunger fillers by Sheaffer)...or to eek out
a little more you use a 1920 system which can fill as much as an eyedropper if
not more as it fills from feed to the very rear of the barrel without the need
of an air space for screwing back on the section as required by an eyedropper.

http://hometown.aol.com/scaupaug3/HARPOON/index.html

Self filling and has to hold a fluid ounce or more? It is the only way to go.