Do you have the authority to waive code at times ? To the point, there seems to be no AFCI breakers available for Federal Pacific panels. Remodel I first saw today will extend BR circuit into sitting room. The inspector I use a lot said remedy would be a small sub-panel. What say you ?
The subpanel is probably cheaper than the AFCI breaker you're going to put in it! You could also buy a small 1-breaker enclosure and place the AFCI in there. There's no issue feeding a 20A AFCI from a 20A conventional breaker.
Would the homeowner benefit from the addition of a new subpanel, or is this a one-off thing?
Also, yes, we do have the ability to waive code if we the installation is safe. 90.4 is a convenient paragraph!