I sometimes tried it. I used the existing rig of the character as MetaRig or I deleted it and created a new on. It was no problem to create the rigify rig an the buttons but it was unpossible to skin this rig. When I tried to make the rigify rig parent of the character with automatic weights, I get the message: "Bone Heat Weighting failed...."
There is an addon by DanielEngler to do just that. Link to download: https://github.com/danielenger/Meta-Rig-for-MLAB More explanations on BlenderArtist if it's not obvious enough for you (it wasn't for me).