A quick browse of the non-Y bucket fare rules (since Y class on BGM-ROM is over $5000k and not what you're looking for) seems to indicate that 1 stopover at CLT or PHL should be allowed for free.
However, I think you'll need to call US to have them ticket this. The US website seems to only want to price it as BGM-PHL + PHL-ROM + ROM-BGM, which as you've seen is more expensive. The number of people taking them up on the free stopover is probably pretty low so having the website support it is also probably pretty low.
It should be easy for a reservations agent to price it and ticket it correctly for you.