In theory it could be done with some careful netcode that avoids all the differences between the console's code and frame rate, plus dummies out the new items in the GCN and Xbox versions...
Basically, no. Sega even requires a separate Hunter's License for each version, and they can't communicate with each other.
Several items (and even entire CLASS CHOICES) were added to PSOe1&2, which make it impossible for them to network with the Dreamcast (and PC) version.
They also added voice chat and a few item changes, as well as several unusual security precautions to the Xbox version.
