Hello David,
I can’t claim to be an expert on this subject but I did take quite a few Saturn images from the 1990s up to about 2011. These were, typically, at a scale of about 6 pixels per arcsecond and 10 – 15 frames per second, using 10 and 12 inch Newts. I don’t ever recall recording Mimas, but I did sometimes just get Enceladus, which is about a magnitude brighter. Like everything in planetary work, it largely boils down to the seeing. In perfect seeing I’m sure Mimas could be recorded without resorting to long exposures of a second or so. In fact I’m sure it must have been recorded on quite a few of Damian’s excellent images.
Expert visual observers (like David Gray, using a 415mm Dall-Kirkham) have seen Mimas when the rings are edge-on and there is less dazzle.
Regards,
Martin