Hey Bunnyboy
I've had a look and no, the lever isn't detachable. It's also got broked at some point as well so probably no good.
It is however just a multi-engagement twist shifter so I see no reason why you couldn't retro fit a friction thumbie or RevoShift twist shifter. Mark up the gear engagement points yourself if you wish too.
I'll have a root round at work to see if there's anything better I can do for you. We do quite abit with IGH so might be able to turn something up.
Hit me up on the email in my profile so I can contact you easier. This thread is getting buried!
Smithy