I have several roku products and bought a roku sound bar and the roku speakers just so I can have them all as fronts. I don’t have any space for a rear. Now my sound bar is useless. I have long been a roku customer. But knowing you will change how my products work after I purchase will make me no longer buy any new roku products. Disappointed!
Not sure that anyone is ever going to see this but I HAD a similar issue. Not specifically about using the center for dialog only but it made no sense to completely bypass the center streambar speakers when you've paid for them. It also made no sense to use the much better wireless speakers just for a little bit of surround sound.
Here's my 99% work around which I haven't seen anywhere else online. This allows me to utilize ALL the speakers that I paid for!
Put the wireless speakers in the front next to the soundbar physically.
Under Setting/Remotes/Speakers set wireless speakers to Rear L & R.
Under Setting/System/Audio set to Expanded Stereo (Music).
This one is key. Open an app like Netflix, anything will do. While playing something, press the * button on the Roku remote and increase Surround Level all the way right to 9 or 10. This setting should stick across apps.
This last one is optional, but you can adjust the dialog setting to it's highest level.
It took me months of frustration to figure this out and I hope it helps someone!