I hope this will be a little bit helpful to you. I am having the same issue. I want to delete very old files from the trashbox.
I found this discussion (by googling, which is how I found your question).
One method is to use CMD window and emulate DOS functions. (if you are doing PC's for over 30 years you will know what this is). The other is to create a script, which is not something that I know how to run, but I will keep looking.
Here is the link: http://forum.buffalo.nas-central.org/viewtopic.php?t=22592