Not so much an exception but a work around according to the guardian newspaper [1]:
[...] the law specifically says “no head and shoulders portrait or bust of any person, living or dead, and no portrait of a living person may be included in the design on the reverse of any coin “created to mark the US anniversary”.
The proposed design features a wider illustration of Trump on the reverse side, a move that legal experts said would fall outside the ban on a “head and shoulders portrait or bust”.
On the US Mint Site I was just looking at the 'Medals', where you CAN have a living person minted onto what they call a medal. Unsure of if it's the same size as a coin, but if it is, you could conceivably try and circulate the medals of living ex-presidents (to great cost to yourself).
There is a law but there have been plenty of exceptions including President Calvin Coolidge on a 1926 half-dollar and Alabama Governor Thomas Kilby on a 1921 commemorative coin and Carter Glass on the 1936 Lynchburg Half Dollar.
