At one point in Prat I hacked up the scrollingmessageframe to try and improve it.
One of the enhancements I made was to lower the alpha on all the lines. Then I could raise the alpha on lines containing text of interest, and they would appear highlighted.
My use case was when you would mouse over the name of the message sender all the messages from that sender in the chatframe would be highlighted, the others faded out. When you stopped mousing over the name the text would display normally.
It was easy to do, you can probably get the code out of an old version of Prat.
By the way. You can also do what you are talking about, and I have some code in and old version of Prat that you could use as a starting point. There was a module called ScrollingMessageFrameHacks I think.
You have one main problem. You either have to dynamically modify the line heights to suit the font, or deal with the visual ugliness of lots of space in between lines when the font size is small.
ScrollingMessageFrame lines are easy to deal with, they just anchor thier bottom to the top of the previous line and stack upwards. Each line of text will grow vertically to enclose the text it contains. You will have to manage hiding lines that overflow the top of the chatframe yourself.
I don't have an image of the highlighting, but this one shows how it is possible to realign the text
I don't really recommend doing this though, because you have to do everything in OnUpdate.