You can do a multipage post by using the
<!--nextpage--> inside the post. The question is:
How can I display the whole post (not paged) for this post.
Well, you can turn it off completely or use the following code along with some sort of conditional statement to switch it on or off.
The multipage part is set up in the
If you hook onto that and then modify the globals that are set up in the function you can undo the multipage config:
Add that to your functions.php or plugin and you're good to go.