I might be wrong but you can't use a selection set to select everything in a circle but you can select everything in a polyline. So you might select all circles as a select set something like (ssget "_X" '((0 . "CIRCLE"))) , then in each one draw a temporary polyline shape that touches the circle in lots of places... and use them as the selection set filter (ssget :cp I think - see Lee Mac ssget function reference for which to use).
i think out there there in the internet there is a function to draw a temporary line inside a circle, but it is Sunday, CAD is of here, so I can't check for you)