you can select that in BBMPEG under Program Stream Setting at the bottom...

I encode the clip normally, with the standard KVCDx3 template settings and VBV buffer 40. Min bitrate is 150 and max 2300/2500 depending on the resolution.

1) I enter the files to the list
2) Select SVCD settings and deselect 'align sequence headers' and 'add SVCD scan offsets'
3) Set the forced mux rate as 0
4) Mux the files and burn as SVCD.

I have to burn it as SVCD because my Pioneer stutters the audio badly even if I've set the minimum bitrate at 700. With an SVCD I'm able to go down to 150 if not even less than that.