daVinci a US maker of tandems, makes cranks, but although they start their range at 150mm, they stop on the long side at 180mm. Until now...
daVinci is going to do a limited run of 200mm cranks. You can get Tandem-Captain's cranks, road double, road triple, whatever you want. However, regardless of what pedal threading or interchangeable spider you select, you can only get 200mm, not any custom size (205mm or 195mm).
However, at $220 these are going to be a steal for custom long Tandem Captain's cranks, let alone road double/triple or mountain triple for 200mm.
If interested visit the Serotta forum thread:
daVinci 200mm Special Order Crank Thread