I have heard of horseback riding on the towpath but cannot find any information on it, ie:which parts it is allowed on accessibility. For that matter I cannot find any information anywhere on the internet about bridle paths on any public lands, though I know it's done. Does anyone know where to get that information?

You may find this link helpful: http://www.nps.gov/choh/planyourvisit/horsebackriding.htm

Horseback riding is not allowed between Georgetown (mile 0) and Swains Lock (mile 16.6) or from Offutt Street (mile 181.8) to the canal terminus (mile 184.5). Horses are not allowed in the Paw Paw Tunnel. Riders must take the tunnel hill trail that goes over the tunnel instead. Riders may not exceed the speed of a slow trot. Riders must dismount and walk their horses across aqueducts. Horses may not cross wooden footbridges, which are not designed to carry their weight.
Your biggest challenge will be to find a place to park your horse trailer. Many parking areas can barely fit a couple cars. For your first trip I would suggest parking at Hancock, MD and then ride the C&O towards Cumberland, Md. This is a nice remote section of the trail (away from the crowds). Hancock has a couple of good sized parking areas. The one that is paved is across from the American Legion. Hancock Md is also right off the highway. Another possibility for parking your horse trailer is near Big Pool, MD and Fort Frederick. Plenty of ample parking at those locations. Also, I should mention that though your question is about the C&O, you cannot ride horses on the paved Western Maryland Rail Trail which runs parallel to the C&O.
I did try to post a web-link about horse use on the trail, but the moderator flagged it. Hope this was helpful.