I'm not sure it would be useful anyhow. I mean, why should one topic line be more conspicuous than any other? Plus, if it were, everyone would just do it, til it was worthless, then some folks would figure out that *not* boldfacing the topic line was actually conspicuous in that situation, and then...
JMHO,
Pat, having read the Dr Suess story about the Sneetches a lot lately (the ones with stars on their bellies and the ones with none upon thars, and the big machine that could star or un-star sneetch bellies...)