An extra line in title style would suffice. It's just extra title information.
As in a release title? That goes in the same place.
The bit about the release artist in the current soundtrack style guide reads like a copy-paste of the style for classical releases. Except that that would only apply to classical soundtracks. Would anyone credit anyone but Vangelis for a soundtrack by Vangelis?
Another case which could just as well happen on a non-soundtrack release. If a guideline is needed for this, wouldn't it be more fitting in a general guideline instead of one specific for soundtracks?
Also, what @221bBS says.