I have 2 x recorded, both Elizabeth built XX7 vans, both 253 4spd and both with Buckskin trim. One is 11/78 so probably ordered off revised colour range introduced on 18/9/78 (Sage green was one of the three new colours introduced earlier than the rest as seen on colour chart A632 released 11/78). The other is 2/79. I also have a 2/79 Kingswood ute recorded, same build config as the Sandmans but has an optional 3.36 diff.
So given the timeframe Sage Green was available vs Sandman end date about 12 months after Sage Green release i'd say it is an uncommon colour, but not exactly rare compared to others.