Mathematically, it is possible. Assuming you have a senior pilot who flies either the 764 or 777, they make $300+ per flight hour. If a pilot flew 100 hours in a month (the FAA max) and all 100 hours were greenslips, he would have an hourly rate of $600 x 100 which would be $60,000. This also doesn't include per diem.

I can't imagine too many pilots would want to do this on a regular basis and I doubt many do. However, given DL's short staffing on the widebodies over the past months, it isn't out of the realm of possibility.

On a side note, greenslipping is generally frowned upon when you have fellow pilots on furlough. Greenslipping allows airlines to run understaffed and slows down the need to recall furloughees. DL's current situation is a little different because of the abnormally high rate of pilots retiring early creating shortages in the widebody flying.