If you want to use the built in IAP mechanism, I think you would have to use Roku billing, and subject to the 30% fee. If you want to build your own however, I don't see it being specifically called out in the developer agreement. Actually, in the agreement it alludes to you being able to let the user know how to get help in the event something is wrong with your paid channel "(whether such subscriptions or billing arrangements are managed by Roku or otherwise)". I'm not a lawyer, but I think that spells out that you can create your own payment acceptance method.
Link for reference
http://wwwimg.roku.com/static/sdk/legal ... eement.pdf
Section 5 Paragraph K
here is no requirement to use our billing system. Many developers do find it more convenient for themselves and their customers to use our billing system, but if you have the technical resources and know-how to create and integrate with your own, or integrate with a third party billing system, then you can do that instead.
We won't let you use our billing system for Adult content, you have to use your own.