You should file that as a “friend of the court” brief.
I was considering one. I think the DC law is fatally flawed because it grandfathers pre-1976 owners who are permitted to possess handguns. By doing so the law violates the equal protection clause, as it effectively discrimates against new residents.
In addition I think it violates the 14th Amendment in that it has the effect of discriminating against (take your pick) women, older people, law abiding citizens, etc. by denying them the right to protect themselves. Ironically the only people who are not affected by the law are criminals, who are free to purchase and posses handguns.
Even though the court has narrowly tailored the question to the 2nd Amendment, I think these argument should be raised as you never know what will sway a justice, or end up as a footnote.
As a side note, the Court seems to focus more these days on empirical data. In this case, the data shows that the ban has been completely ineffective and in fact detracts from public safety. So the supposed rational basis for the law is really a myth. This fact will definitely help our side.