Does the Built-in Bridge (BiB ) recording feature (not supervisor monitoring) support duplication of iLBC media streams?
Yes, most of the TNP and RT  phone models do support iLBC . see ToI in the wiki page referenced below for specific phone  models . If the recorder also  supports iLBC , you are good .

If recorder device does not support iLBC  - it is a code mismatch case . In case of codec mismatch between phone BiB and recorder , CUCM will have to insert a transcoder . CUCM Transcoder did not support iLBC in CUCM 6.x timeframe . not sure what version  you are interested in .

 
Is there anything special that needs to be in the SDP for this on the SIP trunk? (mode=20, mode=30, etc).
Please eloborate on this question . what 'mode' are you talking about here  ?
Please use the below wiki page as reference. As always,  the documents contained in this wiki are Cisco-internal-only and should not be shared with external .

http://cmmonitoringandrecording.gforge.cisco.com/twiki/prod/bin/view/Cmmonitoringandrecording/WebHome