[development] special filter behavior for teaser

I have a client using img_assist to add inline images. They want thumbnails of images to appear in teaser views, and larger images to appear when the entire node body is shown. Others have requested the same sort of thing here: http://drupal.org/node/107801. I was hoping to accomplish this by hacking the img_assist filter to change its behavior when a teaser is being generated as opposed to a full body. But this information (whether a teaser is being generated or not) is not available in hook_filter. Is there any way for me to accomplish this? I cannot think of a clean way so I wonder, should more parameters be passed to the check_markup and hook_filter functions? Apologies if this is a FAQ, and thanks for any advice, -Dave