Ok..I am in the Concorde lounge now and I just chatted with the check in agent about this. Here is their policy:

if you are arriving LHR term 3 on AA first class, then departing terminal 5, you will be able to access Concorde lounge.

if you are arriving terminal 5, then departing AA first class terminal 3, then ‘in theory’ you would have Concorde lounge access, but you will not be able to access terminal 5 when connecting btwn arriving flt term 5 to departing flt term 3. However if your AA first class flight IS departing from terminal 5, then you will be able to access inside term 5 departures, and therefore access the Concorde lounge.

Bottom line, an inbound or outbound flight on AA first will be able to access Concorde lounge, as long as you hv access to terminal 5 departures