Radiopopper’s JrX promises remote flash control with any hot-shoed camera. It uses UHF frequencies to avoid the line-of-sight requirements of optical remotes. But there are mechanical problems with some cameras.
The JrX transmitter’s antenna is a little knob protruding from the bottom, clearly designed for an SLR’s pentaprism shoe location to raise it above the camera’s body. On cameras with flat tops, you can’t slide the JrX into the hot shoe because the antenna blocks it.
What you need is an extender with a hot shoe on top. I had an unused shoe-mounting Wein Safe-Sync lying around. It did the trick nicely on my Leica M8 and M6 TTL.
It works on my wife’s Canon G10 too.
So yes, there is an off-the-shelf solution when you want to use a JrX with your Leica M or other flat-topped camera.
If you’d rather go the DIY route, a replacement hot foot and shoe can both be had fairly cheaply. Metal Supermarket can supply a small piece of aluminum stock for an extension. They can also recommend a local machine shop to tool it for the foot and shoe attachment points, and the hole for the ‘hot’ wire.
Find the nearest Metal Supermarket at