In one of his videos Henry Lincoln says the following:

The Virgin Mary with Child on the Crucifix is not a common sight, but it is by far no means unusual.

As for the example that exists in Rennes-le-Château, this could be simply paying homage to the Basilica of Notre Dame de Marceille, dedicated to the Virgin Mary, situated not far from Rennes-le-Château. On 14 September 1862 the Coronation of the Miraculous Statue of Notre Dame de Marceille was observed; that is, on the Feast Day of the Exaltation of the Holy Cross. In 1887, the celebration observed its 25th Anniversary. So within this particular context, this could be the explanation for the grave(s) in Rennes-le-Château containing this specific imagery.

Below are some more examples of this imagery, with or without the Child Jesus. We are still looking for anything found in Rennes-le-Château that cannot be found anywhere else.