^Agreed with CWS.
Re-checking in PHL after arriving internationally is very easy. Just veer to the right instead of going into the arrivals hall on the left.
I'm based out of PHL. I'm BA Gold, I use the BA lounge all the time on domestic flights... Have not been turned away with an AA boarding pass for years (and I think that was at Silver). It definitely depends on timing and capacity of the BA flights for sure.
I've always thought the lounge staff were extra pleasant and accommodating... especially loose with the allowance of folks allowed on the left side of the lounge if you have friends/coworkers on the right
The AA lounges in PHL are much more spacious. If I'm not looking to get a nice buzz going, it's off to them, usually.