Well don't have pics to prove it but.
Took my Contax S2 on my back while cycling the Alps in 2001 (Celerina), 2002 (Davos),2005 (Canazei), 2006 (Chamonix) 2008 (Courmayeur) , 2009 (Cogne) and 2010 (courmayeur again). Climbed the Grand San Carlo, the Petit Saint Bernard, Col de l'Iseran.... Used the standard ER case but from a nikon FM10 (better than contax one) and a neoprene strap. Carried the camera with the strap on my left shoulder, crossing my chest and passing under my right shoulder/arm, camera back was resting on my back with lens pointing upwards.
Sometimes you have to stop to replace the camera in the good positiion so that it does not go bouncing around.
After 2016 I used exclusively my Zenit 122 (which survived to a magic tarmac crash at 24 mph (40 kmph) when the plastic bag I was carrying it into broke and fell on the tarmac. HAving seen such resilience my Zenit is the bike camera.
Had previously used a Canon Snappy 20 in 1997in the austrian Alps (Galtür), again in 1998 in Switzerland (Champery), 1999 (Kandersteg) .
So a Rangefinder Leica with a large neoprene strap and a good ER non leather case would be no problem
Best regards