New in libguestfs upstream and 1.21.39 is the ability to access disks over FTP, FTPS, HTTP, HTTPS and TFTP (read-only).
You can use it like this:
$ export LIBGUESTFS_BACKEND=direct
$ guestfish --ro -a http://x.x.x.x/scratch/winxp.img -i
Welcome to guestfish, the guest filesystem shell for
editing virtual machine filesystems and disk images.
Type: 'help' for help on commands
'man' to read the manual
'quit' to quit the shell
Operating system: Microsoft Windows XP
/dev/sda1 mounted on /
><fs> ll /
total 1573209
drwxrwxrwx 1 root root 4096 Apr 16 2012 .
drwxr-xr-x 23 1000 1000 4096 May 11 18:45 ..
-rwxrwxrwx 1 root root 0 Oct 11 2011 AUTOEXEC.BAT
-rwxrwxrwx 1 root root 0 Oct 11 2011 CONFIG.SYS
drwxrwxrwx 1 root root 4096 Oct 11 2011 Documents and Settings
-rwxrwxrwx 1 root root 0 Oct 11 2011 IO.SYS
-rwxrwxrwx 1 root root 0 Oct 11 2011 MSDOS.SYS
-rwxrwxrwx 1 root root 47564 Apr 14 2008 NTDETECT.COM
drwxrwxrwx 1 root root 4096 Oct 11 2011 Program Files
drwxrwxrwx 1 root root 4096 Oct 11 2011 System Volume Information
drwxrwxrwx 1 root root 28672 Oct 11 2011 WINDOWS
-rwxrwxrwx 1 root root 211 Oct 11 2011 boot.ini
-rwxrwxrwx 1 root root 250048 Apr 14 2008 ntldr
-rwxrwxrwx 1 root root 1610612736 Oct 11 2011 pagefile.sys
Apart from being a tiny bit slower, it just works as if the disk was local.
Richard WM Jones 
Presumably, this only works well on HTTP servers that support byte range queries?
Probably. It’s implemented using curl, so it does whatever curl does. Is there a web server which doesn’t implement byte ranges?
Sadly, there are “security” middleboxes acting as semi-transparent proxies that stomp on byte-ranges. My employer uses one of these. I found out by trying to use aria2c to grab Fedora install disks =).