Here's the twist: I need the channel's appId before I can submit a complete iOS app to Apple for review, but I found no way to get the appId info for a channel that is not yet public (I submitted the channel to Roku for review - but it is not yet public).
I tentatively side-loaded my channel via https://my.roku.com/account/add?channel=[mychannelcode], and queried my Roku device for installed channels (see below for query results). However instead of getting a Integer as is standard for appId, I got "93580_4d63" - which I figure is probably an interim appId, not the actual appId of the channel after it becomes public.
Amusingly dev.portal does not show you direct the channel#, the most important property there is (it'd be the "table primary key" in SQL parlance). But if check the URL when on your channel details, it will be something like