Does Ripple support subadditive trust?
Ripple doesn't support anything like this directly. You could get the effect if you really needed it though. Simply create a new account and extend the amount of total trust you want to that new account. Then have the new account extend the individual trust to each of the other accounts.

This will mean that people paying you with IOUs from any of the accounts in the group will have to ripple their payments through the new account you created because you won't accept those IOUs directly. You will wind up with IOUs from the new account regardless of which IOUs from the group they try to pay you with and your new account will hold the real IOUs. So the credit you extend to the new account will limit the total amount of IOUs from that group you can hold.

When you make an outbound payment, you'll go through the new account and reverse the process, trading in the IOUs from the new account for the "real" IOUs.