NO.

ATL is layed out: all in a long line:

Domestic terminal : Concourse T / A / B /C / D / E / F / International terminal

It is fully connected airside.. But you have checked bags, so must exit security to the domestic terminal. At which point you can not reenter. BA does not have a desk at domestic, so you must circle around to the international terminal via a shuttle. It can not be walked.

EDIT: This is specific to the problem you have where you are departing from an airline with no domestic presence. If the flight was on DL you could check back in at domestic and walk. You could try to see if AA would check you in for BA (as AA has a domestic ticket window), but I thought this came up on another thread and somebody said unlikely.