as the models where probs being paid by a non-japanese company (or more likely not paid atall) i doubt the japanese immigration or tax departments will be too bothered.
Doing a bit of casual DJing or MCing would probs not be worth the effort getting a work or entertainer visa in my eyes. The work would probably be low paid and cash in hand unless your a top level DJ or entertainer.
If you've atually got solid nights booked and your a professional entertainer then sure, get an entertainer visa, if your going over to check around or on a verbal 'yeah sure, you can spin a few at my club' promise just treat it as more pleasure than work.
Oh and bearrock, if you come across any cool turntablist clubs or fresh graff sites, give me a holla yeah, im heading over early next year and looking for cool places to check out.