[plt-scheme] detecting a here-string syntax object

Is there any way to detect that a syntax object is a here-string? For 
example, Scribble leaves enough breadcrumbs in the syntax properties.

More specifically, is there any way to get the source location 
information of the first character of the string contents? I'd like to 
produce syntax errors with proper source location, but all I have is the 
location of the initial '#' character, not the location of the actual text.

Thanks,
Dave