Welcome! Glad to hear you’re enjoying learning Scrivener.
It’s worth noting there is already an existing feature request for allowing use of the <$img…> placeholder in these fields, so what you’re trying to do isn’t possible anyway at a technical level. All it supports is lightly formatted text.
More to your query though, the placeholder help file has this to say about placeholder usage, under the Header and Footer subsection:
Not all placeholders are supported in headers and footers. In addition to the special header and footer placeholders specified below (which can only be used in headers and footers), only the placeholders listed under the sections Page Numbers, Current Date and Time, and User and Project Information are supported in headers and footers.
So, it doesn’t matter if the ToolGlyph field has an image reference in it, or the text “BlahBlahBlah” typed into it, it cannot take document level metadata and insert it into the header or footer, for reasons noted in this older thread.