Wait a minute... I know this is old, but as I'm looking up more of what I need to make this happen, I'm seeing on the thread you linked a few months ago:
They're talking about an SD Card option over the Broadband adapter for emulation on the console itself. Is this legit? Because I can't find that dang Dreamcast BBA for less than 100 USD
You can use the SD card adapter to emulate an ISO, though you'll need to reboot your console constantly and navigate DreamShell each time you modify, add, or remove content from the ISO or change your program. I'd recommend using an emulator and intermittently test on a retail unit.