I searched on LexesNexis, but only came up with cases regarding suits by firearms owners against states like NJ which made legal owners of mags criminals overnight unless they surrendered their equipment and stated so in a letter telling of the date and what was being given up.
Additionally, it seems that the LEO Mags would only be an issue if turned up in the commission of a crime. None of these cases as Robert pointed out are enforcement against guys like you and I, but don't chance that they wouldn't come after you, because they would if you stared down their barrel.
As a side note, I did find an interestig case in California (People v. Dingman, 47 Cal. App. 4th 1068) where the defendant converted a fixed mag SKS into a removable 30 round type and was successfully prosecuted, even though the 1994 bill and Cal penal code states that the rifle was to be manufactured with a detachable mag. Interesting case, I didn't realize that California had such a shitty body of law.