BGA adapter boad?

I have a customer working on a new design of a rather large board that needs to use a micro BGA that will require 8 layers. He needs to use that BGA and it does not come in any other package. He asked me about making the BGA an adapter or daughter board so he could make the rest of the board 2 or 4 layer to keep the cost down. This would also eliminate the need to do micro-BGA level rework. The part would be attached to a small board with a grid of round pads on the bottom side spaced so that we can have access with a HP-3070 tester and the mother board would have the same grid. After the module was tested it could be placed on the mother board like a large BGA but with paste only-no balls. I think this can be accomplished with a 6mil stencil. If we use the outline of a large PLCC we can use old matrix trays to present the part to the machine. Has anyone ever tried anything like this? It seems like it would work great. If the micro BGA does not pass test we just pitch it and it would be hard to imagine many problems with a BGA-like module with 100mil spacing having too many issues being soldered to the main board.

Any feedback would be greatly appreciated even if only theoretical.

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