Do what a compressor does, the manual way: Adjust the global volume.
But first off, get your mix right. Give each instrument a good volume so that it sits nicely in a mix. Attenuate higher notes played by the same instrument, as they will come out louder just by nature. Yes, this may require that you manually enter volume commands next to half of your notes, but it's so worth it. Also play around with channel volume and instrument/sample volume to balance things.
After that, you can export your tune to a wave file (or simply into a sample slot in OpenMPT) to check where are the loud and the quiet parts. Decrease the global volume in the loud parts, and slowly fade it up and down when transitioning between parts of different volume.
You can see this technique being used e.g. here: http://modarchive.org/index.php?request=view_by_moduleid&query=177409
If you just want to limit your module to make it loud and sacrifice its natural dynamic range, there is no alternative to a compressor, but you shouldn't do that anyway (welcome to the loudness war
). Manually adjusting the global volume is completely okay though.