I think this is a job for a collapsible lens for the M2 rather than a new body.

An M2 with a 2.8 Elmar is size wise on a par with a CL and 40mm Summicron, what one looses in one direction the other makes up for in the other. Weight wise there is about 300gm in it, favouring the CL, but in terms of slipping a camera in your (coat) pocket the M2 and Elmar beats the CL. True the CL has a nice meter, and I love my CL, it is a nice camera to use, but a thin lens for the M2 is far more fun. If not a Leica lens check the hand made MS Optical compact lenses available from Japan Exposures, the 35mm Perar is a superb little Cooke Triplet design.