IDK if this is true, but something that I have observed.
Members of the Embargoed Alliance who can trade with me fit into one of the following categories -
1) They are VIP members
2) They have joined the Alliance after it was embargoed by me but have accepted the trade offer after joining the alliance
3) They have changed nation color or bought a new city
I do not know which one holds true, but some direction for Sheepy to think on.