I said I'd update/add pictures, but obviously never did....
I changed my approach, bought a HoHey Stunt Handbrake Junction Valve Adapter from Moto Madness in the States (as recommended by dean0n0 on another thread). This was expensive for what it was ($60 + delivery) and took a lot of chasing before they sent it. Don't think much of their service. Very slow response to emails.
Anyway, fitted it in conjunction with a long hose from eBay and a front brake m/cylinder and lever from a Honda Silverwing scooter (£14 used off eBay). Hose long enough to avoid having to remove the tank to route it.
This works well for what I wanted, which is something to steady the bike up, not a full-on rear brake (just in case I forget that it's not a clutch and haul on it). Because I still have the foot brake lever, I can use that if I want more rear brake power.
It feels a bit weird if you use both at once, as the pressure pushes back. So, if you have the LHRB engaged and press the foot lever, the bar lever moves out against your fingers.
I can also use the LHRB when backing the bike down my steep drive past the car to the garage. The front brake just locks the wheel and the bike continues to slide back! On a conventionally geared bike, you can have it in gear with the engine off and use the clutch as a back brake.
I've got a Bullet parking brake on the front m/cylinder banjo bolt. Also from the States and also expensive, but their customer service is very good.
The CRF450 rear m/cylinder and Smokin' LHRB set-up went on the GasGas EC300, which has a Rekluse clutch.