No, if Richards refuses a minimum tender offer from the Spurs and chooses to join the D-League (theoretically speaking, it has never been done), the Spurs would retain his rights.
There is only one unsigned draft pick I remember playing in the D-League, Ricky Sanchez. His was a fairly unique case as he played for the Idaho Stampede when it was a CBA team, so his presence there was kind of a grandfathering situation. No other team could call him up, though. NBA teams couldn't call up those who were going to be in the draft after the season either; e.g., Latavious Williams, Mike Taylor, Jeremy Wise.