What if the firstbits are superposed on the picture (normally shown 6 letters or so in the bottom of identicon, without leading 1) to make the match harder? So both reading and abstract pattern recognition is engaged at once.
Replying to myself again 
, but just for fun if we disregard prefix we can fairly easily match 32 bits:
1KbhFQVEUk8wMVtiuBURZAQ1PXnsDUqcag
1EEwcrjaJkWLLZDuZA2Rhob3aVM8NwY5tR
or 40 bits:
1NcE7wksPMcydG7bfsGsGdjf2ckzXSfw1R
1H26EaqCrbdHZk2SvqvZDfPHqYGbYqQsJj
Not going to try for 48 bits on the CPU, but with OpenCL code on a GPU it shouldn't be bad either.
, but just for fun if we disregard prefix we can fairly easily match 32 bits:1KbhFQVEUk8wMVtiuBURZAQ1PXnsDUqcag
1EEwcrjaJkWLLZDuZA2Rhob3aVM8NwY5tR
or 40 bits:
1NcE7wksPMcydG7bfsGsGdjf2ckzXSfw1R
1H26EaqCrbdHZk2SvqvZDfPHqYGbYqQsJj
Not going to try for 48 bits on the CPU, but with OpenCL code on a GPU it shouldn't be bad either.