I managed to get this working with the following settings:
Encoder: C:\Program Files\Windows Media Components\Encoder\WMCmd.vbs
Extension: wma
Parameters: -input %s -output %d -a_codec WMA9STD -a_mode 2 -a_setting Q50_44_2
Display name: wma
Tick the Hide Console Window setting.
It does not transfer any metadata and there are three digits inserted into the filename, but otherwise this does a great job of converting to ~60-90kbs wma files - which is fine for jogging along busy roads.
Thanks for the help.