Glad to hear that a legislative solution is underway.
Should a judicial avenue be required, the best plaintiff, in my humble opinion, would be a resident of Vermont, Illinois and/or a U.S. Citizen residing in Canada, and who spends time on occasion in Michigan (i.e. a 2nd home). Add to that, a Plaintiff who has a clear need for self-defense while traveling (disabled, elderly, single mother, etc.). To further back the State really into a corner, the Plaintiff should also be dependent on public transportation i.e. bus, tram or subway (thus the Plaintiff would have no practical means to comply with the challenged law in the first place - due to lack of an inaccessible storage place such as a trunk).
Without being able to establish legal residency in Michigan (not legally possible/practical to have dual residency), the sympathetic Citizen Plaintiff faces a total and complete ban on possession of a functional firearm within vehicles. A complete ban on possession of a functional firearm for self-defense purposes failed to satisfy any level of scrutiny noted the Heller Court.
Securing a win there would all but assure that loaded, readily accessible, vehicle carry would then also be legally available to all law-abiding Michigan residents, regardless of CPL status.
I applaud the legislative effort as it is probably more resource efficient at this time. The litigation avenue may not be as ripe as it could be since the Supreme Court hasn't yet ruled on the right to 'bear' outside the home yet. That could change as early as next year. Regardless, as it stands now, the ban on loaded, readily accessible, vehicle carry is surely in violation of the Michigan Right to Bear Arms constitutional provision as well as the Federal Constitution's 14th Amendment equal protection clause that is supposed to ensure that all Citizens, no matter what state they are in, are afforded the same rights that any other State citizen enjoys.