Will there be an access panel? You can not bury the old receptacle/junction box behind a wall.

or a fireplace either, for that matter.

Could this be considered accessible since the box will be in the fireplace area; not really hidden (like junction boxes above a suspended ceiling)?

UHHH... ALERT?!

I'm seeing two proposed options:

1- the back of the fireplace is between the fire and the box. The box is hidden. Can't hide boxes.

2- the box is in the fire.

I think a good way to start an electrical fire... is to put an electrical box IN fire.

I sincerely hope I misunderstood something somewhere!

It's a faux fireplace.

The outlet would be covered with a plate and could easily accessed by simply removing the fireplace shell (couple of screws). The existing box will be behind the plywood back of the faux fireplace surround, but there's a 3" gap, it won't be touching anything nor be inaccessible.

I'd really like to know if the wiring I proposed is correct.

If the plywood back can be easily removed to access the old box then it is acceptable. If the plywood is being permanently nailed in place then it is not acceptacble.

"If the plywood back can be easily removed to access the old box then it is acceptable."

Access must be available by removing screws (no nails).

You can make an access panel in the back of the cabinet for the old box.

"My plan: kill the power and run a new wire from this duplex location to the new outlet in the surround, installing a blank plate over the existing wall duplex with a hole drilled out of the center to hold the new romex."

If you do decide to go this route (and can keep the junction box accessible), instead of getting a blank cover and drilling a hole in it, get a cover with a knockout in the center and install a romex clamp.

The link below shows a cover that costs about a buck at a hardware store. This'd still not be the tidiest of wiring jobs (IMO) but it'd beat a wire poked through a hole with rough edges.

Here is a link that might be useful: