Hi!
This hopefully should be an interesting job :)
What you require certainly is possible with the help of clever tricks using jquery and javascript but whether or not the themes' insertion point is enough or not, that's hard to say. So, your point
"I require this to be done with no core or plugin modifications - the work must be carried out using only settings in my theme or in wp-admin" may not be valid in terms of plausibility. Editing the template files may be necessary.
The reason I am bidding on this is because if you'll check my reviews, you'll see that I am particularly good with coming up with solutions that others can't.
Hoping to hear from you!
Have a good day!