The web editor was originally designed to input or edit songs and make playlists, not specifically to extract, but this procedure might work.

After importing the complete playlist into the web editor, Copy all the code from the Forums box and paste into a basic text editor (Notepad or Text Edit for instance) on your computer.

You will be able to work out each song as they start at the beginning of a text line and the beginning of a song starts with:
[ url=irealbook://xxx
and this song ends with:
xxx and the name of the composer.
(where xxx is the name of the song)

Delete the songs you do not want above and below that song.

Delete from the beginning of your song:
[ URL=

Delete from the end:
] xxx [/URL] by yyy
where xxx is the title of the song and yyy the composer name
- note that you have to delete the closing square bracket ] just before the title and then to the right of it at the end of the composer name.

So now it should start with:
irealbook://xxx
and end with the sign:
%20

Copy that text making sure you do not add extra spaces at the front or (more likely) at the end of the code (just take the text as you see it and nothing more.)
Open a new web editor (called New Song), Paste the code into Import box and click on Import button.
In theory this should be your single song.

You will probably want to verify it works by emailing to iReal b on your device and testing it opens, plays, transposes etc. (of course there may be faults in the original song in the playlist so you will need to check all is working.)

No guarantees this will work, but hopefully it will.