Why on a Mac?
I dont have the specs on any in-boom furling but I would suspect that weight would be a problem, but not your biggest one. A few years back I considered an IN-Boom furling system for my Catalina 30. The precision of installation can be quite a nightmare if not done properly. If you raise and lower your mast regularly, I dont think it would be a practical solution. I realy dont know much about them though and sometimes new technological developments cure all of the old ailments. My real question to you would be.....WHY? The canvas on a Mac is small enough and light enough to be easily handled. ALSO>>>>>>>>>>>>>>>>>The added weight of an IN-Boom or mast furling system will certainly throw a light displacement boat like a Mac out of balance by raising the center of gravity. Could lead to a roll-over situation. Besides, any furling system you add to a Mac, other than on the jib, defeats the purpose of simplicity of the Macs. You either should add your own lazy jacks or buy another boat.