I'm responsible for 2 very, very old games (Jumpman and Beyond Castle Wolfenstein for the PC) to be open sourced and released, but it's reverse engineered source code and not original. I contacted the original authors (although in the case of BCW, Silas Warner had passed away, but his widow gave me permission). These games had passed through so many hands by the time I got around to them that the rights had simply been lost to bigger companies who didn't even know they owned them. Not only were the rights gone, but the original source code was always long lost.
Hopefully someday the original source may materialize in some box of floppy disks in the back of a closet (see jordan mechner's recent discovery of Prince of Persia) but the odds of these things are so incredibly rare.
About all I learned was that this is tough to do, very time consuming, and it's easier to ask for forgiveness than to get permission.