Hi H.,
The only thing I can think of that might work for her at this age is.... is there something she wants something big like a new barbie doll house, maybe even a puppy, a kitty, a hamster, a fish or something that you would be willing to get in exchange for the thumb sucking. In a way it's bribery but at least you both would get what you want.
I watch a little boy who is 6 1/2 months old now and he sucks his thumb also "chews" on it since he is teething he has made the inside of his thumb raw and bleed, to try and break this his mother and I have noticed that he sucks his thumb when his gums hurt, he is hungry or tired so we try to make sure and distract him when we notice these things. We have started putting socks on his hands when he goes to sleep so he also doesn't suck his thumb. Just to not have this happen. So far he hasn't figured out how to get the socks off and he still doesn't really care for the pacifier.
Is it also possible for her to wear a glove or even a pair of socks over her hands during the day that way when she does go to put her thumb in her mouth there will be a barrier. I have no idea if this would even work but it's just a suggestion.
I also watched a little girl that would suck her thumb everytime she had her favorite bear here or even when she had it at home. Thats the only thing I can think of is if you notice her doing at certain times like sleepy, watching t.v., or hungry or even boredom. I hope this helps you a little W.