A Hassle. Someone else should answer your question directly that's in the know. In general, I would recommend not hassling with it. Simon's service www.lagniappe-belize.com is much easier and a better value when you consider any food restrictions and/or shopping on your own.
I second Otter. Go with Simon, at least for starters. Bacon is great from the Sausage Factory, just inland from Xanadu. There are a bunch of fruit stands all over town, with the freshest available. Why bother trying to bring anything from home?
This is the way I'd do it. Pack a small or medium carry on with nothing but frozen food packed tight. Don't say nothing about the bag and if you loose it so what. Somebody will eat it. Nothing ventured nothing gained.