The problem isn't the manufacturers' willingness or not to make a base, it's the size of their paper machines that is the limitation. You can make 500kg of paper on a mould made machine, but a Fourdrinier machine may need several multiples of that - an individual 1600m master roll is about 500kg of paper at double/ premium weight. If people were prepared to pay Art 300 prices ++, then more mould made surfaces could probably be made. Personally, I'd like a pure linen hot press base (naturally high white point, and linen is about the ultimate for archival quality), but I'm under no illusions as to what it would potentially cost - especially as coating packages need adjusting for getting non Baryta base papers to work well with modern coaters.