Posting here for my reference, but others may find this useful.
2800bps, avi encoding:
mencoder.exe %1 -oac lavc -ovc lavc -lavcopts abitrate=128:vbitrate=2800:acodec=mp3:abitrate=128 -ffourcc DX50 -o %2
2800bps, two-pass avi encoding:
mencoder.exe %1 -oac lavc -ovc lavc -lavcopts vbitrate=2800:vpass=1:acodec=mp3:abitrate=128 -ffourcc DX50 -o %2
mencoder.exe %1 -oac lavc -ovc lavc -lavcopts vbitrate=2800:vpass=2:acodec=mp3:abitrate=128 -ffourcc DX50 -o %2