I know others have done similar things. I have some stupid, fragile render hook code that allows you to misuse the image title attribute.
This markdown:
![Alternate text](a.jpg "Title text {id='foo' class='bar'}")
![](a.jpg "{style='border: 1px solid #ff0000'}")
Produces this HTML:
<p><img src="a.jpg" class="bar" id="foo" alt="Alternate text" title="Title text"></p>
<p><img src="a.jpg" style="border: 1px solid #ff0000"></p>
I can post if you’re interested.