since the remote it default to BT or RF it will not control the playbar. i found this article
[Link: support.sonos.com]

but it describes setting up sources first (DVD, cable box, ect). My customer ONLY uses the apps built into the TV.

Will this process lock the volume to IR for all sources (including apps) or just sources that are connected?

I did the exact same thing for a client a few months ago, they only used the apps also, it worked perfectly.