Are there some programmatically (ExtendScript) accessible pproj metadata

I am trying to store some information (textual json, on smaller side) directly to `.pproj` file metadata (via ExtendedScript), but I cannot find if it is actually possible. 

I managed to do it in AfterEffects and Photoshop via `app.project.xmpPacket`
(In PS it is then visible under File > File Info)

I was looking for something similar in Premiere, I found this:

but it seems that are metadata about imported files only.
Example on Github always points to some `projectItem` https://github.com/Adobe-CEP/Samples/blob/master/PProPanel/jsx/PPRO/Premiere.jsx#L1000 , but I would like to have access to metadata (to store and retrieve) in completely empty `.pproj`.

Stored metadata aren't necessary user editable, but I would need them to be persistent from one launch to another.

I might need to create some 'sidecar' json file living next to `.pproj`, but I would rather like to have it directly encapsulated in `.pproj`.
Would it be possible to import or create some 'invisible projectItem' and use that one for storing metadata on that level? What would be appropriate data type for such projectItem?

Any comments or feedback welcomed. Thanks.