I'm no expert, but I was under the impression that yoyogi is kind of a free-for all. Of course now that I think about it, You may need some kind of permission if you are drawing power for amps and such, but if you were acoustic or unpowered you can do whatever you want. Lots of people practice instruments there (horns and the like), just because its a space where you can be loud, as apposed to thin walled apartments.
In any case, I'm sure if you were to get permission of some sort, you could play whatever you wanted, whether they are your songs or not, and really, how would they know anyways?