Noisebud has released Leaky, an effect plug-in that leaks the side information into the mid.

Noisebud Leaky

Leaky will let you leak the side information into the mid channel. It can be useful while mastering if parts of the audio almost disappear in mono.

REMEMBER: Taking the track back to the mix should be your number one choice, tricks like this should not be used if not absolutely necessary!

The plug-in is available for download for Windows (VST) as donationware, with a minimum donation of $9.90 USD.

More information: Noisebud / Leaky