This is great! Don't see much necro in /lit. I'd like to see Mort get a shot at some rare meat like one of the three princesses other than Twilight [not that Twi shouldn't end up on the slab eventually too!). Maybe it'd be easier to write a scene like that for Chrysalis instead, while this series is still new.
How about she was executed by beheading after her attempted invasion, and Celestia wants both her head and body taxidermied and exhibited as an example to any would-be usurpers. As for clothes, I'd imagine she'd be stripped for the execution, but maybe she was still in a queenly dress. For her post-mortem outfit, if not nude, maybe something degrading like a whore's outfit.