It adds that behaviour by default to Scamper and GeeMail. The key is in the TextComposer composeEachRectangleIn: rectangles method. I have added an instance variable called baseline. If the value is 0, the behaviour is the normal behaviour in Squeak (bottom alignment of text in lines), if its value is 1 the text is aligned in the middle of the line.
Neat work!!
For full Scamper support, it looks like it would be enough to modify the method HtmlImage>>addToFormatter: so that it sets the baseline setting....
-Lex