Same issue. I have banks in France also which require 6 and 8 digit PINs.
This makes MasterPassword unusable for me at present, however I am truly attracted to the concept.
Do you plan to allow the length of the PIN to be defined by the user and, if so, by when?
In the mean time, you could just repeat the sequence to fill the missing digits.
For example, if the generated PIN is 1745 then you do 174517 for your 6-digit PIN.
Who would guess you're making your PIN's that way? ;)