I've been turning this over in my mind, too...how to accomplish the job without using a lot of storage space is key for me.
I'm contemplating using the regular portable propane grill for the meat chamber and building an add-on out of stovepipe or something that could channel smoke/heat into it at a controlled rate. This is how many combination BBQ-smoker setups work, and it seems to me that something could be cobbled together to do the same job. The grill wouldn't be connected to the gas, of course, and since it wouldn't be getting high fire temps I'm not worried about damaging any of the gas components. And it'd do two jobs instead of just one.
My grill doesn't have a hinged lid, so the idea was I'd build a "collar" that raised the lid enough to allow for a cutout for the pipe...firebox itself to be an older portable, guts removed.