After some experimentation, Evan Millner arrived at the following variation of the standard English Heritage Spec. Mortar Mix for architectural reconstruction: This mortar mix would be suitable in any hot climate.
lime 1 part
stone dust 1 part
sand 1 part
1 teacup of seived wood ash
Wood ash as pozzolan is not problematic in the Barbados climate, due to the constant heat. (Wood ash generally absorbs water, and in for example the damp cold English Winters, would ruin the plasterwork).
I examined plaster on a number of ancient buildings around the island over the course of my stay, and all the plaster observed had traces of wood ash clearly visible, sometimes not even particularly well seived. Some of these plaster surfaces were hundreds of years old, yet had survived well.