Well, the BC clearly intends to upend the status quo. Maggie LeFay was an idealist unhappy with the status quo. Those two facts certainly don't prove a relationship, but given Maggie LeFay's popularity and notable skills in the wizarding world, the other major backers of the BC probably would have considered her a useful recruit/dupe.
In addition, Maggie LeFay is known to have had a change of heart at some point that considerably rearranged her motivations. After that change of heart, she planned to have a child with Outsider-bane properties. Considering the BC's known links to Outsiders, a connection starts sounding pretty plausible to me.