You certainly can, but you run the risk of stalling the bike unless you are able to simultaneously pull in the clutch lever...But for simply scrubbing off some speed, sure.
You could. MotoGP has been doing it for years. The biggest problem in the past was having enough room on the handle bar for both the hydraulic clutch and the brake master cylinders. It looks like the new Rekluse MC could fix the problem.
Thumb brakes have been around for years. At least 20, if memory serves, on Mick Doohan's GP bike after his horrendous crash at Assen. Not a bad idea, but I'm not ready to drop $325 on something that I may or may not be able to exploit.
I have the new Rekluse master cyl lhrb and use it in conjunction with the stock clutch mount. Neither are in the ideal position, but I am able to use a single finger on both very effectively. I like it, the price was high, but the unit is small and had a great lever.
So after reading many threads on the idea I installed a left hand rear brake on my 2013 250xc race. I can tell already its going to be a game changer, love it!
I purchased the pc that you drop in the rear master cylinder from Rekluse for about 108.00, the lever (same one that Rekluse uses) from Ebay for 20.00 and then bought some hose. in all it cost me about 160.00 and took under hour to install.
I had a severe dirtbike accident over 20 years ago where I shattered both wrists. My left wrist had surgery with pins and an errector set attached to the outside. My right they casted for 8 weeks.
At 42, I'm beginning to get some painful arthritis when I ride. I'm thinking Flexx bars or Rekluse to ease that and make life easier.
I've been a mechanic for 25 years my hands and wrists hurt when riding my 500's with a stock clutch set up(all have been converted to hydraulic). I race my GG and after installing the rekluse my hands/wrists don't hurt and I'm much much faster in the really tight single track. It's like cheating to me, but at my age(44) I need all the help I can get.