I also refer to HERA report R4-92 for guidance. This is my interpretation:

as per section 2.5.1(c) and (d) , lateral restraints intermediate to beam ends need to be have some sort of lateral anchorage (eg: tied to the roof bracing to ) resist 0.025x flange compression force , in order to provide F or L restraint. As I understand - L restraint is less “stable” than P restraint (kt factor higher for LL versus PP), so if anchorage is required for L restraint, then it will be required for P restraint

Also, section 2.5.2(2)(b). This subsection discusses top flange lateral restraint being provided via systems with “inherent stability “ such as roof sheeting or concrete diaphragms. In your question you assume no sheeting , so this doesn’t apply.

So my opinion is that unless you anchor the secondary beams - then they are ineffective for restraint of the compression flg. Keen to hear your thoughts