given how the tablet unmounts the cards when you plug in the cable, its likely to be some limitation in linux.

Nah, not a Linux limitation, mostly a spec, time, implementation and testing sort of deal. Nokiaprobably did't see it as a feature worth investing a lot of time in at this point. Which isn't unreasonable, as the implementation isn't particularly simple, and there isn't a lot of space to do thing with on internal, anyway.

Besides, there are plenty of ways to get to the rootfs without involving a USB cable, sftp or sshfs and FUSE are two very good options.

Because Linux was not written for phones, it has to unmount the cards. This is because sane people do not want two OS's to write different data to the same place. So, the best way would be through sftp, because this is wireless (aka over WiFi), and its easy after you set it up. After installing OpenSSH on the tablet, I think you can use the built in "Connect to Server" dialog in OSX. Not very sure, because I have not used a Mac since school ended. In Ubuntu, the "Connect to Server" has sftp support, so probably OSX does as well.

Note: I know that you want to connect over USB, but this is over WiFi.

What I said is basically the opposite of this. You can use your Mac to access the tablet, instead of the tablet accessing the Mac.

When the tablet is hooked up to a PC via USB, it presents the memory cards as USB mass storage devices. That means the PC gets raw device access, below the filesystem level. So, the N8X0 cannot write to the card while hooked up via USB, since it'd confuse the attached PC and lead to data corruption.

It's conceivable that the N8X0 could maintain read-only access to the memory cards while hooked up via USB, but I expect it'd require a fair amount of programming, since two computers (N8X0 and PC) would be accessing the same raw device.