Unfortunately - as of 1.4.2 - Scribus only allows for very simple TOCs.
However you might like to look here
http://forums.scribus.net/index.php/topic,919.msg4158.html#msg4158 and here
http://forums.scribus.net/index.php/topic,323.msg1485.html#msg1485 for some discussions that might be of some use to you until this changes.
I believe a general recommendation would be to leave the "finishing" of the TOC as your final task. I.e. Add the attributes as you go, generate it when you want to check its size etc, then when the rest of your editing has been completed generate the TOC for the final time and edit it as you need it (making sure you don't regenerate it afterwards).
It's not an ideal solution.
P.S. In the meantime, if you can come up with a more detailed specification of what you want you can always post it as a feature request in Mantis and see what other people think about it. You never know, someone might already be working on something similar.