I don't know about full time, but sometimes when a client asks me to extend a piece of software I'll ask the upstream maintainer if they'll accept a patch for t

I don't know about full time, but sometimes when a client asks me to extend a piece of software I'll ask the upstream maintainer if they'll accept a patch for the change (with permission from the client of course). The client gets software with the fix included, where they don't have to worry about checking each future upgrade to see if the fix broke something.