Type2 loader entries are just UKIs[1]. I don't know what you are implying with "systemd-boot's automatic sourcing of UKIs isn't a BLS thing"? Any way I try to interpret it, neither would the Type1 entries fall under the definition of automatic sourcing (neither anything any other bootloader does).
Which ones? If you want to all you need is a single UKI in /EFI/Linux and everything is synthesized from that.