I really don't want to have to resort to using their last used withdrawal address. 
This could be really bad. Some people could have an inputs.io address as their last withdraw address.
Agreed. I'm not sure what else to do. Things have to be brought to a close. (And the servers are still costing $$/mo)
There are also a bunch of accounts that never did a withdrawal, so for those I am likely going to put together a donation to the Bitcoin Foundation or something similar.
The good thing I guess is that the majority of these are not large balances. Most of what's left is less than a single coin.