It's all moot for bikes anyway-- I found a site selling nitrogen cartridges (same form factor as 12g CO2)-- used to push wine out of casks, and from their text, "Please be aware that the nitrogen will only fill 20% of the space that a 12g CO2 cartridge will because Nitrogen exerts a lower partial pressure than CO2." So a nitrogen cylinder capable of filling even a 700x25 would be like... fist sized.