I think I've discovered another issue: It seems like REAPER refuses to open any LV2 UI which requires (in its metadata) the `ui#parent` feature. If this requirement is removed from the metadata, the ui can be opened successfully, so it seems REAPER *does* provide this feature, on macOS at least.
You can test this by modifying the ttl of the distrho mini-series plugins (
https://distrho.sourceforge.io/plugins.php) to require `ui#parent`.