Klaus,
If I understand it correctly, you want to put a citation from the clipboard into your FM document by pasting, but the pasted text should have double angle brackets around it to make it show up as a citation. I would think that the easiest solution would be to have a script for "Paste as citation" instead of trying to tweak the clipboard contents before pasting it. The "Paste as citation" simply pastes whatever is in the clipboard to the text location and then adds the double angle brackets. Adding the "Paste as citation" command to the Edit menu is not a big deal and you can define your own shortcut to invoke the command.
Would this not be much easier than changing the cilpboard contents ?
Good luck
Jang