I am a Canadian and I camp there all the time. Do you have a friend or relative with a US address? We use my sister's address in Maryland even though we have Ontario plates.

I don't see why it would not be available to foreigners, however you may need to contact the water district in person to have them take your reservation if the website won't allow it.

Have you tried to call the office to see if you can reserve a campsite via the phone? (352) 796-7211 In the past, I have found them very helpful. As for other free boondocking/dry camping locations I am not aware of any others that specifically support those without a US Address.