I don't expect any problems with the police. We're 100% within our right to have an open carry picnic, and this is 100% legal. I know responsible gun owners and 2A activists enough to know we'll be a peaceful and well behaved group, so there will be no provocation and no cause for concern from the police.
Then again, there is always the situation my friend encountered when protesting Mitch McConnell because he twisted arms in the senate to pass the TARP-1 bailout for Wall Street banksters even though 92% of his constituents were against that redistribution of their money to those crooks.
While my friend was being frisked for no good reason, I was about 100 feet away, carrying concealed, and another friend of mine was open carrying in the Fayette County Republican Headquarters at McConnell's speech! He wasn't hassled, but after the event some guy who was there told me that he shouldn't have open carried because it made people nervous. I wouldn't have open carried there, but my friend was completely legal to do so unless asked to leave by the people who own or rent the building.
If you go to an open carry picnic, bring your video camera!