This can be done, but it must be done manually. Get your Spectramatte key the way you want it and then select “Show Alpha”. Alt-apply your BCC Drop Shadow effect as normal and select “Apply to Title-Matte and Invert Matte”. Then, step into the BCC Drop Shadow effect and on the layer underneath the Spectramatte key edit in your original source footage. There will be three layers underneath the effect. The top is the Alpha, the middle the source, and the bottom the background. Then step back out of the nested effect as normal.
It also important to remember for this effect to work you’ll need to have an empty layer between your nested effect and your background layer.
ac