I am back with another question on resource guarding

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So - Wesley has been making very good progress at home. He can be called off even a delicious RMB - he will come to me for a treat and then I can go get the RMB while he sits and waits... and I can usually go over and take the RMB, give him a treat and return the RMB. The success rate continues to rise as we continue to work on the resource guarding protocol suggested in "Mine!..."
What has stumped me is the unexpected acquisition of a high value item (ex - in the park this morning someone had a squeeky toy - not against the rules, but IMHO a terrible idea... Wesley got it, took it to a corner and guarded it. The woman went to get it from him, I told her not to, that I would do it, I approached and as I reached down and said "give" - he growled. I told him "ah ah" and then backed away, called him, told him"drop" which he did - I praised to high heaven and gave him a treat I had hidden in case of emergencies). I guess my problem is that I knew that there was a chance he wouldn't have given me that toy (especially with other dogs around - which always adds stress...). Any ideas on how to practice and train for that type of situation?