I was under the impression that this was a dangerous thing to do, because by using custom filtering you implicitly accept responsibility for what illegal content slips through your filter.

I can't edit my post anymore, but I couldn't find a source to back this up. According to the Tor Blog, it should be fine to have a reduced exit policy.

For reference: https://blog.torproject.org/running-exit-node

I know but I was referring to identity part;

'in order to ensure that deterrent effect, it is necessary to require users to reveal their identity to be prevented from acting anonymously'

How do I avoid Tor exit nodes that block "abuse" as defined by copyright barons?