J.,
There might be additional buckles out there you can add to your seat. I know if I asked my husband, duct tape would be the top of his list. :) Still, a single strap with buckles on either end might be available - and could be buckled to the side of chair where she can't reach. Duct tape over the buckles might work now that I think about it. But the real problem is the safety issue, that you need to tackle too.
I explained this to a 4 year old who was in my care at the day care I worked at: the principle is the same, though the actions were different.
"Part of my job is to make sure you know how to keep yourself and your friends safe. I need to know you are going to be safe, I need to know you are going to do the safe thing, and I need to know you are going to keep your friends safe. You cannot do that while you are doing (x,y and z). This is what you are doing that is unsafe - how can you make it safe?"
I also am starting my son early on trying to make/help him understand restrictive belts. We use the slogan "click it or get a ticket!" When he is older we will explain more, but for now it is an expected thing, and he sometimes leaves off his car-seat dance to let me get him in there without exhausting myself in the process. (Even before he could sit up, he was noticing how we put this thing on and copying our movements for unbuckling!)
Another idea, but it would require a little ingenuity to make it work, is to go through the motions of getting a ticket. What do we give up? Money. What would a child have to give up? It would take some thought, I think, because she is too young to understand money, and toys are best friends right now - you wouldn't pay a ticket with your best friend. So how to make this idea work would require thought on your part for what would work best for your daughter.
Good Luck!
M.