Custom Entity on Package Part Structure

Hi

I have an issue for one of my customers (running 22R1).  They have a requirement to add a new Custom Entity to the Package Part Structure page for a Sales Quotation.  

The problem here is that the Page is using the Entity OrderQuotationLineMaster, but this is not available in the drop down list for the Parent Entity within Entity Associations on the Projection Configuration (QuotePackageStructureHandling):

Without this option, I cannot build the association between the 2 objects to be used on the Page.  If I use OrderQuotationLine instead, then the relationship cannot be used on the Package Part Structure Page.  The Navigation Property only exists in the Projection metadata against the OrderQuotationLine entity, not OrderQuotationLineMaster.

Has anyone come across an issue like this before?  How can i build the Association between my Custom Entity and OrderQuotationLineMaster?

Thanks

Gareth