The Dahon Mariner D7 that we currently have in stock is 135mm OLD by my measurement (without removing the wheel). We recently upgraded a Mu P8 to a DualDrive with an 8-speed cassette. Here's a list of the items for the Mu upgrade:

20" wheel with DualDrive hub (built by Quality Bicycle Products)
DualDrive 8-3 shifter
DualDrive click box
X4 8-speed rear derailleur
Derailleur Hanger (to accommodate a standard rear derailleur on the Mu - this might not be possible with the Mariner)
Housing for gear cables (~9')

Expect to pay $450-$600 for a local bike shop to do this upgrade.

-HANK RYAN-
Norman, Oklahoma USA