We flew from Texas to Belize in 2004. We hugged the gulf coast around MExico and then straight across just north of the Guat. border. You must have over-flight permission prior to flight for Guatemala if you chose to over-fly. When we flew into Belize we had a handler named George Gillett--he is very good and made our entry paperwork go smoothly. The only unfortunate thing is that the aviation authourity did not allow us to fly wherever we wanted to in the country. Our main destination was Placencia area, but we wanted to also go down to Punta Gorda and up to Corozal etc. They told us that every time we wanted to fly somewhere new we had to go through Belize City and clear it with them, which of course was totally inconvenient. I don't know if they were concerned about the possibility of drugs or contraband being transported, or if it was a safety issue of some other sort, or what. I think that the next time we go in our plane (we've been a few times commercially since then), we will probably be able to get permission to go a few places without having to "check in". The other bad thing is that the only place you can purchase aviation fuel for private aircraft is at BZE or the municipal airport. Which really sucked because while we were parked at the Placencia airstrip, a lot of our fuel got stolen; they left us just enough to fly back to BZE. Another thing: after you leave BZE it is all CTAF. Which is OK, but there is a lot of traffic what with Maya Island Air and Tropic Air going regular routes all day and the pilots all are mostly speaking Kriol, whcih makes it hard to understand who is where etc. If you call out your intentions in English though, they will respond in kind.
Plus, aviation fule is expensive, so be aware of that. If you have a Garmin 396 or better that is good so you can check out your terrain avoidance going through Mexico.
Any more questions ask. I had a blog about it on blogger.com btu I dont' know if it is still up.