I did go on the trip, and it was the highlight of my week on vacation. The museum is in an old Cotton Belt shop building, owned by the UP but lent to the Cotton Belt Historical Society. The building is the same where the 819 was built. Since it's a shop building, there's a transfer table outside for moving equipment around. The diesels I can remember are as follows: The Cotton Belt GP30, a UP C630, two Alco "alligators" (one RSD15 and one RSD12, IIRC), and a GE U-boat which was parked outside on the day of my visit. Supposedly they also own a small GE switcher, but I don't remember seeing it. There are no rides or regularly operating equipment - the location is surrounded by only UP-owned tracks - but as soon as we walked in a volunteer gave our group (three people) a grand tour, giving us access to locomotive cabs and such along with a wealth of information.
The building is pretty huge, so along with the rolling stock there is also some model railroad equipment, a display of different types of rail, a retired CTC board, and probably some other things I can't remember. The first railway museum I had ever been to was IRM, and since it's so huge, the first thing I usually think when I've been to a smaller one is usually "oh, this is it?" That never really happened at the Arkansas museum - it's small but extremely interesting. Plus it's indoors so you can go in the rain.
Oh - and about pictures. I did take some, but unfortunately my scanner is not capable of scanning slides very well. Hopefully I'll have them online sometime in the future but right now I only have the images on slides.