This would result in an increase in server load over the current AOP method. At the moment, it opens a text file to check for last modified shouts, completely avoiding any database connections or any other vB related overhead. Doing MD5() would, at best, simply produce an increase in the time it takes to check for new shouts (having to MD5() content left and right vs. checking timestamps) and at worst, re-adding the vBulletin overhead.
I doubt very many people would pay to host this content on CDN, especially since it would be an increase in page load times for end-users since it's slower to connect to a CDN vs writing AOP files to local disk.