I have the same issue where I can set the attribute size to be anything in PDMLink, but am crippled by CREO to 80 characters. I contacted PTC TS a couple weeks back and they were unable to provide a solution or work around.
I am wondering if Pavel Burtsev's idea could be used somehow though. If the attribute was not mapped to PDMLink it seems like it would work, but could you get a larger (than 80chr) PDMLink attribute to pass down to the note? It seems like PDMLink will just overwrite what ever is already there truncated to 80 characters.