There is a product called a top hat adapter that you can use to run both a brake disc and a sprocket on the left side.
Things to consider though:
Is the rear wheel's hub already set up with ISO brake disc mounting holes? This is necessary.
Does the bike already have the mounts for a rear disc brake caliper?
If not you can use or make an adapter.
Is there enough space on the left side to accommodate both the sprocket and and the disc/ caliper and still allow proper chain alignment?
Here is Dogtown Burner's thread about his Grubee GT2A framed build. Read through it and you will learn of his experience with running both a sprocket and a disc brake on the left side.
http://motorbicycling.com/showthread.php?t=42432
Every bike frame is different and not everyone's solution will fit another bike frame.
Some will be easier, some harder.