If anyone else is interested, the answer was very simple.
You just use:
uuid BLABLA (no quotes needed around the value)
NOTE: You must use the exact case for the letters in the uuid value! It makes a difference between small and capital letters (as Linux anyway).
So if pointing to a windows partition I found the following working:
uuid BLABLA
chainloader /bootmgr
NOTE: bootmgr is used for Vista and 7, previous to Vista it was ntldr (if I am not mistaken). Haven't tested with /ntldr because I needed the value for windows 7 anyway.