The source surface is available from the EffectConfigDialog.EffectSourceSurface property after the OnLoad event.
This is how I got the info for the histogram in my 'Threshold' effect. But, as Rick pointed-out, this may not help much if you effect is complex.
If the effect is a pixel-op, you could extract a thumbnail image from the source surface and then duplicate the thumbnail once for each preview image, appply a different effect to each...