As you note, seasonal agricultural workers are very important to the ongoing work we need to do to supply food to Canadians, so we work very actively with those countries from which they come, to be able to identify the various means by which they could make their way here. However, it's the employers themselves who are actually very good at trying to ensure that these workers can make their way to Canada.
For example, yes, they are using existing flights, but another way in which an employer can ensure that seasonal agriculture workers come is via charters. Charters are sometimes put in place so that they can come from Mexico, Honduras or the Carribean, as you mentioned, directly to Canada.
Despite the restrictions we currently have, temporary foreign workers and seasonal agricultural workers are deemed essential to the supply chain, so all necessary elements are being made for those who have the work permits in hand, to be able to make their way to Canada.