Doesn't the S1 model (04-06) have a helmet lock (uses ignition key) on the left side just below the seat?
For my 07 (picture #1), I added a Yamaha lock assembly to the right side so I now have two(for a pillion).
Picture #2 is a Genuine Yamaha lock cylinder (part # 26H-21308-01-00 ) and is re-keyable to your ignition key.
If the S1 was does NOT have any locking tab to mount a lock assembly to,
if there's interest, I can make up a frame clamping model (which I already make for
BMW's-uses a BMW lock).
Pictures 3, 4 and 5 are for a BMW, round frame(just larger thhan the FZ6), GS1200 and 1300. Picture #4 is for a right and left side, same bike. You want the bracket situated so that when the lock opens, the HOOK, holds the "D" ring and keeps from dropping the helmet.
I would just have to change the frame size to fit the FZ6 and lock pattern to fit the Yamaha lock.
Some other BMW lock assemblies I designed and sell thru
Pirates lair:
The Easy Helmet Lock for the BMW K1300S, K1200S, K1300R, K1200R, & R Sport Any of his
'EASY HELMET LOCKS" are my design which I manufacture.
BTW, for Nolan helmets with NO "D" rings, the "T" shaped helmet lock extension, can be slipped into the strap and then used with a regular helmet lock.