Needs new forks anyway, hence the question, however no desire to create something for the sake of it. Have 26" forks to put on anyway so may keep in normal format.
Idea works best with a suspension corrected 26er frame and a rigid 29er front end that should fit nicely with little affect on handling. Same idea pretty much with a half fat.
Historymans mad 69er in frame form now resides at Velo towers and will be staying in 69er form but with a shorter A-C rigid fork.
Front benefits of a 29er, that is the ability to ride over more stuff plus rear benefits of strength and quicker acceleration.
Disadvantages more noticeable if you stick a sus fork into mix as stated by others. Rigid is way to go with ghetto build 69ers