Can you manually draw a curve opposite to the HPF with any of the plug-ins in your DAW?
If you cannot manually draw a curve, it may be hard to reverse the HPF with exactness but you should be able to 'kinda' do it with a shelf and/or notch filter if you play with the settings (especially including the octave/width or "Q" settings with a notch filter).
It may be best, however, to just apply a similar HPF to the other channel, especially since a -12dB/octave HPF at 40Hz really should only be cutting out rumble and the very low end of the frequency range that most playback systems cannot (accurately) reproduce anyhow.