marketing constraints!
Pity the poor manufacturer and all his starving employees. The perfectly good helicopter is flying well and selling well, when managment 'listens' to the calls for increased range. "Sure, we'll increase tankage by 1/3!"
Out comes the new model. The Press gets to fly it & 'critique' the performance. What's this? The new model is SLOW, and has very limited altitude capability, and almost no cargo capacity! "What a dog," the press cries.
Bingo, another bankrupt helicopter manufacturer! All because The Press chose to fly it with full fuel, as, "of course," all the operators would do. Or so they believe.
Ergo, I don't see a lot of market for bigger tanks. The ferry pilots, of course, a)have no voice in the matter, and b)are frequently time-builders, anyway. So the manufacturers have little to gain by increasing tankage, and a great deal to lose!
[One brilliant application for the R22 optional (?) aux tank: Showcopters has replumbed theirs to carry the airshow smoke-generator-oil. 'We don't need no stinkin' avgas!']
Sorry to be so negative, Heady, but you may need to develop an STC for bigger tanks all by yourself. Or you could emigrate to the USA, where landing fees are a rarity and the 'trackless wilderness' is absolutely LITTERED with MoGas outlets. All tended by lonely farmer's daughters.
Dave