The CSharpLang github[1] repository would be where to look for this. I didn't find anything that mentioned adding language support for it, and IIRC, in the past they rejected the idea of inline XML or JSON support.
Going the other way of C# inside HTML, you have Razor[2] and Blazor[3], the latter of which is a prototype of Razor running in the browser via web assembly.
Blazor is pretty amazing, but my coder fantasies mostly revolve around a React-JSX-in-C# vision - the syntax and approach just feel so natural and so right. Your point about rejecting inline XML is probably a pretty good data point that I shouldn’t expect what I’m hoping for to happen.