As you said, since the IFA max is not officially part of the game, there is nothing built into the game to break IFA ties. I learned the hard way that it's whomever has the max offer in during the cycle he signs. i.e. if the cap is $15M and you offer that and are in the lead, if someone subsequently offers the max after you, they will take the lead.
The reason for this WIS explaining to me is that IFAs are programmed to try to get as much money as possible, so even though subsequent bids only "tie" yours, the IFA will tell you someone else is in the lead so you'll up your offer. Now, this was a few years ago, so I can't be sure it wasn't changed, but I doubt it.