jmf,
I am not familiar with hawthorn thorns but I am all too familiar with thorny honey locust. I'm running 6-ply industrials on my Toolcat with slime or slime equivalent added by the Bobcat dealer. I've not had any tire issues so far and don't recall seeing any thorns broke off in the tires.
FWIW, after dozens of plugs in the Gator tires I added slime and never added another plug but you could see numerous thorns broke off in the tires. I've also added slime to the front tires of the 4010 ag tractor after several thorn problems and have not had a problem since. The rear tires have never had a thorn related leak.
I will be purchasing a pair of ATVs in the near future. I will have slime waiting for them.
dsb