Did you ever figure this out? It looks like Brightcove was trying to sell this feed generation capability as an add on, based on our brief email chats with support. We've been with Brightcove for years. They kept on wanting to schedule us with a business development rep., which usually indicates there's added cost.
If you want to work with Direct Publisher, check the DP forum for other channel developers who have software tools available to assist.
Roku Direct Publisher JSON feed needs to be accessible without authentication.
That said, take a look at our OTTfeed platform, its all-in-one feed management system for Roku Direct Publisher and custom OTT apps (March only: price starts from $50/month).
For more info go to https://ottfeed.com/ and request a demo or PM me.