Can the Second Amendment reasonably be construed as applying to slaves?
If a state has the authority to make someone a slave as punishment for committing a particular crime, that would suggest the authority to put whatever restrictions on the person's actions it sees fit.
If a state wished to impose a law requiring that, as punishment for committing a particular crime a person shall be (unless or until pardoned) a slave of the state who shall ,after serving a certain term of years in prison, generally be allowed to move around freely but will have certain restrictions placed on his actions, including a prohibition on weapons ownership, I would see no constitutional problem with that.
That is, however, a far cry from the federal government's GCA'68 which forbids firearm ownership for people convicted in state courts for crimes where they could receive over a year sentence, regardless of the actual sentence given.
I don't think Brady et al. would want to use this rationale to justify the prohibition of firearms to convicted felons, since it would equate people who are forbidden from owning firearms with slaves. While that may be the agenda, I don't think it's one Brady et al. would admit to.
If they are imposed only during the time they are a "slave", then yes that would apply. But Congress shall have power to enforce this article by appropriate legislation. would not allow them to apply after being released from "slavery". I also think that GCA and NFA are both unconstitutional, but because of the infringement not because of who or how it applies.