I would think a spade bit would give you the counter bore for the screw head and washer you're talking about. But I would be wary of the rest of the plaster just disintegrating and falling down anyway.
Also, if it's plasterboard the strength is reliant on the paper staying intact.
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I would think a spade bit would give you the counter bore for the screw head and washer you're talking about. But I would be wary of the rest of the plaster just disintegrating and falling down anyway.