This is unfortunatley not possible, because when you render a montage with the batch processor, there are less options available than when you render from the Master Section dialog. More exactly, you can only render the montages as a single file, not as multiple files.
The only approaching solution I see would be this one:
1) you render each audio montage as a big wave files with marked regions, using the batch processor. Each one in its own folder.
2) you use the auto split clip (it has a batch mode), to split all these wav files in smaller wav files.
3) you use the batch processor again, to convert all wav files into mp3 files.
That's not so much work actually because each step #1 to #3 is a single big batch step.