I thought there were a number of Presbyterians who argued the Mosaic wasn't an administration of the Covenant of Grace. But, most 1689 LBCF Reformed Baptists I've come across believe it was a gracious covenant. Sam Waldron argues the confession teaches it.
The Particular Baptist teaching is that the Mosaic Covenant is a mixture of both the Covenant of Grace and Works. It is a stand alone Covenant that ministers both on some level. Some believe it is neither but subservient to both Covenants. I believe that is John Owen's understanding per his Commentary on Hebrews Chapter 8.