Perhaps a forward slash could be used as the start and end marker, so long as it is preceded by a space or is at the start of a new line? Visually, a forward slash is slanted like italics.
Valid: I am a member of the species /homo sapiens/.
Invalid: No space, so this will render the forward slash normally: homo/sapiens/.
Modified examples from the HTML5 spec page:
The /Felis silvestris catus/ is cute.
The term /prose content/ is defined above.
There is a certain /je ne sais quoi/ in the air.
<p>The <i>Felis silvestris catus</i> is cute.</p>
<p>The term <i>prose content</i> is defined above.</p>
<p>There is a certain <i>je ne sais quoi</i> in the air.</p>
Dream sequence example:
Raymond tried to sleep.
/The ship sailed away on Thursday/, he
dreamt. /The ship had many people aboard, including a beautiful
princess called Carey. He watched her, day-in, day-out, hoping she
would notice him, but she never did./
/Finally one night he picked up the courage to speak with
her—/
Raymond woke with a start as the fire alarm rang out.
<p>Raymond tried to sleep.</p>
<p><i>The ship sailed away on Thursday</i>, he
dreamt. <i>The ship had many people aboard, including a beautiful
princess called Carey. He watched her, day-in, day-out, hoping she
would notice him, but she never did.</i></p>
<p><i>Finally one night he picked up the courage to speak with
her—</i></p>
<p>Raymond woke with a start as the fire alarm rang out.</p>