With some coaching on the IRC, I got this working.

The trick, it appears, was a combination of using this Injection Selector:

L:(text.html.markdown $)

along with this rule set (only markup.raw.* are important here):

{ patterns = (
{ name = 'source.yaml.front-matter';
begin = '\A-{3}';
end = '-{3}';
patterns = ( { include = 'source.yaml'; } );
},
{ name = 'text.html.markdown.classes';
begin = '{';
end = '}';
},
{ name = 'markup.raw.block.markdown';
begin = '^`{3}';
end = '^`{3}';
},
{ name = 'markup.raw.inline.markdown';
begin = ' `';
end = '`(?=[ .,;-=+])';
},
);
}

Bob
-------------------
Robert J. Rockefeller
Richmond Hill, GA
www.bobrockefeller.com