You could use a second individual entry template for this.
Just put all the design that you want it in with only the basic tags in it -
CODE
<MTEntries><$MTEntryExcerpt$></MTEntries>
This will be generated for each entry you make.
Then use an archive filename (in Blog Config under Archiving) to name them differently than your normal Individual Archives - something like this:
CODE
<$MTEntryID pad="1"$>_extend.html
and then in your Main Index Template, you'll want to add some code so this will popup -
CODE
<MTEntryIfExtended>
<span class="extended"><a href="<$MTBlogArchiveURL$><$MTEntryID pad="1"$>_extend.html" onclick="OpenComments(this.href); return false">MORE</a></span></MTEntryIfExtended>
This will open it up in the same sized window as popup comments would.
Hope this helps!
Kristine