If the filesystem is mounted, it can be used to expand the size of the mounted filesystem, assuming the kernel supports on-line resizing. (As of this writing, the Linux 2.6 kernel supports on-line resize for filesystems mounted using ext3 and ext4).
Question: (I'm a bit of an old fart and used to administer SunOS and Solaris systems; I don't have a lot of Linux experience so my terminology may not be correct.) Would it be possible to specify the character device for the disk instead of the block device? My experience is that programs that complain about systems being mounted (or not) may work with the character device if the block device had been specified.