That sounds like a good compromise, since if something goes wrong in the planning stages and I can't make it, you could easily revert it to plain old (ha! unlikely!) Bedtime Stories. Half an hour sounds manageable.
(For a little context, I'm going to be leaving Japan on the 3rd of August, flying straight to Tennessee to meet my boyfriend's (huge) family, a week later flying from there to Seattle to see my family and unpack/sort/store/U-Haul/mail my things down to California, driving down to California around the 20th, moving into a new home, and starting my orientation at grad school on the 23rd. I don't want to come off as a flake by refusing to completely commit, it's just going to be a month of unadulterated crazy is all, and I don't want to inconvenience folks by signing up for a bunch of stuff and then falling on my face just short of the finish line.)
That said, I'm going to do my absolute best to be there. For a half-hour filk panel, maybe I could do just three songs, a little talk about the writing process, then brainstorm ideas with the attendees for a simple group-written filk to a well-known song? If there's a whiteboard, we could write the filk on it as we go, then all sing it together at the end.
The cosplay idea is something I've wanted to do ever since I started attending Japanese conventions. Both sides of the Pacific have overinflated ideas of the other side's expertise, treat foreign cosplayers like rock stars when they show up at domestic cons, and harbor a bit of an inferiority complex about their own stuff--it's interesting. There are also a lot of freedoms that American cosplayers take for granted in our daily antics. It might be better to wait until Sakuracon or similar, just to be sure I could pull it off, but I've been snapping enough photos of cosplayers and shops out here that I think I could do a pretty good slideshow presentation at some point.