That’s just a matter of syntax, could be #field:value in text instead, just like #tag is no more a standard in markdown and also kind of conflicting with the header format, and hierarchical tags are not a standard thing either, those are all choices.
As to parsing variable in a whole text, that would only happen when the frontmatter is changed, and the whole thing is already parsed again and again as you type for syntax highlighting and formatting, it is basically what CodeMirror does, and that library is used for syntax auto-completion etc, things far more demanding than markdown.
Anyway, this thread is more about functionalities description that “how to implement them”, those are functionalities that exist in other software and that I find incredibly convenient, and ideas should be discussed on their merit and usefulness and not on how difficult they would be or conflict with a standard that needs to be extended anyway to offer anything more useful that the basics, that’s why markdown gets so many flavors and that so many stuff is already represented as codeblocks. If markdown gets in the way of anything useful, maybe it’s not the right tool for the job, or maybe it needs to be turned upside down
Doing standard for the sake of doing standard is much less important than having the tool be useful for the user, and that in my opinion is the only thing that matters.