Well, so far, CDex uses the following strings to pass onto the external encoder:
%1 is the source filename
%2 is the destination filename
%3 is the bitrate desired
You really don't need to use %3 unless you want to encode using CBR or ABR and wish to specify the bitrate within CDex as in the External Encoder settings tab I showed above. For VBR, leave it out.
For LAME 3.98, the following HA Wiki gives you a list of all the command line parameters it accepts:
http://wiki.hydrogenaudio.org/index.php?title=LAME