Most Dreamcast browsers don't support the BBA, and most games don't, either. PlanetWeb 3.0 is the only US browser that works with the BBA, and it should use the same settings as XDP. PSO and Quake are the only games that work with the BBA, and while some people can get online with PSO using the XDP settings, other people seem to find that it doesn't work. For those people, selecting the "Website" option on the PSOv2 title screen and configuring the BBA there makes PSO happy and work with the BBA. Note that PSOv1 may complain about the BBA not being supported initially, but once you configure the BBA with USv2, PSOv1 will let you use it.
It is also worth noting that PSO requires you to configure a dial-up ISP to play online, even if you have a BBA. If you haven't already done so, you need to remove the BBA, connect the modem, and use a browser like PlanetWeb or XDP to enter a fake username, password, and number to dial. Once you've saved that information to the flash memory, you can connect the BBA again. If you don't have connection details for your modem saved to the console, PSO will tell you that you must configure an ISP before you go online, even when you're using a BBA. The game isn't smart enough to know that the modem details are useless with the BBA.