i think u misunderstand.
well it shouldnt really add lag to any great degree, as i assume its a cheap operation (as in, it will always stay within the same frame, ie wont skip frames)
AA works but taking a larger resolution image of a screenshot + then selecting X number(or so) samples within that image to come up with a final pixels value eg 4xAA == sample 4 pixels in the same region to make one pixel, (usually how its done now is only on the edge pixels + ignoring of the textures only depth values)
thus what u ask for is impossible.
take a 1280x720 + somehow turn it into a 1280x720 4xAA image even with inifinte time, the only way to get from the 1280x720 to get 1280x720 4xAA 4xAA is to have acces to better data the what they recieve