I have read in older threads that Immigration at PHL can sometimes be crowded and time-consuming.
As I will have to make a connection within 90-120 minutes (according to the arrival history of my inbound int'l flight) on a Saturday in 2 weeks, I guess there is somewhat of a chance that I could miss the connecting flight.

Even though I do fly quite a bit, I think I have a stupid question:
If I was too late after Immigration and Customs to make the connecting flight, what would be the usual procedure to take care of getting booked on the next one?
Are there dedicated US Airways transfer desks after Customs or would I go to a US Airways ticket counter or somewhere else?

I will have checked luggage, and will arrive int'l at Terminal A and connect to domestic Terminal B (US Airways to US Airways) and travel in Envoy/First - if that is of any matter.

The checked luggage will limit your options, but here's what I'd do:

If you missed the connection because the incoming flight is way late, then you'll likely have been automatically rebooked. In that case, head to any check-in desk with your luggage.

Otherwise (that is, if youmiss the flight because of Customs/Immigration delay), first choice is to call US Airways and get rebooked over the telephone. If when you try this it looks like you'll be on hold for a while, I'd also start moving to the ticketing/check-in area. In that area, there is typically one or two desks that are marked as "Ticketing." Line up at the ticketing desk, and use whichever one (phone or ticketing line) opens up first.

If you didn't have checked luggage, then your best bet would be to go through security (you'll presumably have a boarding pass, and 99% of the time that boarding pass will be ok to get through security, even though you may have already missed the flight). Then, head to the lounge -- you'll have access regardless of status because you're traveling in Exec/1st -- and let the lounge agent handle the reticketing.

Excellent.
Thank you very much for your in-depth info.
Very much appreciated!

Guess I got lucky. Time from deplaning, thru immigration, baggage claim, customs, baggage re-check today: 20 minutes! Now I got enough time to discover that PHL isn't the fanciest of all big airports. But who cares as long as everything is on-time