Hi Anne,
I definately understand the frustration you are experiencing! I have been in similar situations. Like you say it will help this 1 dog in particular but it does not help the next one & address the real ploblem: that these people are ignorant!
If the owners will not freely give you the animal I would call the law enforcement officials. The laws are not great here either but as a law abiding citizen you are obligated to at least try to go through the proper channels first. Otherwise you may get labled as an extremest & a trouble maker & the plate may be reversed & the law may side against you!
Any ways, good luck. I hope you can come to a solution. Have a good one.
Linda