UML-wise, adding or removing a "drive" without reboot is possible. That is, if you take care to properly unmount it before removing. All you need is access to uml_mconsole, which you don't have on Linode

You might be able to ask caker to add that functionality to lish or web interface; It won't hurt to ask.
As for resizing, reiserfs filesystems can grow online, without the need to unmount. Shrinking it is not that easy though. Linode's kernel 2.6.7 has reiserfs support on (don't know about 2.4 series). So you see, there IS a use for online resize after all
