You can get special Muck Boot Rider Boots (I think that's what they're called!). Muck Boots are so the best things on the planet - if you can justify it in any way then do! They're fab!
When muckers were first brought out they were advertised as suitable for riding and yard work but there were a number of accidents where the velcro straps because wedged against the stirrup resulting in the rider being dragged. If you look at the product descriptions now they shoudn't mention riding.
They are the most fantastic, warm, comfy wellies for around the yard - that's how I justified the cost to my OH. Wouldn't fancy riding in them though as they've got no heel.
The muck boots for riding in are the Tyne model and they are fab. I've worn them for the past 3 winters and they have really changed my comfort - I have bad circulation but my feet stay lovely and cosy in these.
the muck boots that are suitable for riding are called the tyne muck boots or check out the Just togs version mudruckers they are a lot cheaper and just as good
I got the tyne muck boots last winter and totally recommend them as they're the only boots to keep my feet warm and dry and last the winter! Although they're suitable to ride in I found they were too bulky to ride properly in so didn't use them for schooling or jumping although they were OK for hacking.
I have the riding version and they're fab!! I school, hack n jump in them, go XC schooling, have even competed in them! They're just the same as my old mountain horse boots to ride in.