Mag locks on market defeating the law already for AR owners, essentially making them no different than any other fixed mag semi auto.
Youre idea has been tried to ill effect. No effective means to positively feed the ammunition without redesigingthe mag, which requires atf approval and paperwork(creating new ammunition feeding device). Complexity, cost, durability, etc all come into play. If it doest feed reliably you wont sell asingle unit beyond protos.
Could i come up with something, yes, but i have already lost far too much money and time to this law. There are other lawsuits going through after reg begins and im waiting until then before i tool up for stuff thatwill be needed.
Right now you can make your AR compliant to the law an not have to register via a maglock withnothing more than $5 worth of hardware from lowes and maybe that much of your time, but you then have to load from the top via takedown.
Im staying out of the gun game for bit aside from coatings and custom bits(stocks, parts, etc)