Alright, I had some free time to tinker around with it some more, and found a Web Browser that works normally on my NTSC-J console: http://www.theisozone.com/downloads/dre ... rt-finale/
That was a bit of success, however when I boot my Japanese copy of PSOv2 (along with the Sylverant PSO Patcher disk), it still shows the warning, that I have no ISP configured - despite saving the ISP information to DC flash from the browser mentioned above. It shows the error while trying to go online, or access the in-game browser. I'm using the original Japanese GD-ROM.
Any idea? Do Japanese games work only with the ISP settings from a Japanese Dream Passport? I remember setting up the ISP on PlanetWeb browser on my PAL Dreamcast, and it worked just fine.