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 As a Canadian author and manager with extensive negotiation experience, I think that advertising by either party during negotiations might influence the outcome or transfer a feeling to any side of the table that the result is planned ahead of time.

However the basic of advertising fundamentally aims to highlight a specific perspective, often powerfully leads audiences to the advertiser's point . 

For example, using a video of President Reagan’s speech as an advertisement by the Canadian Primer could act better as a defensive tactic in proper timing , but not right before or during a negotiation. 

This is the same senario as political ads are paused before elections—to avoid influence or lead or mislead the results.