This could be pre calculated and stored like a cache and updated every time, just like it already happens with other things (like rank updates)... I do not think that we have any problem if there are a small delay in the correct count.
There wouldn't even be a small delay. Theymos can do this by database scripts:
1) Create new ignore_plus_trust score column in the member database
2) Populate over time
3) Call Increment/Decrement sp each time ignore or trust is modified.
The only people this would concern are:
1) Those are ignored by a lot of people
2) Those who have extraordinary negative trust
Don't limit protection to protect a few social rejects.