The best linux is the one your most comfortable with.  The details of which packages, and package managers are included by default are much less important than your ability to work smoothly with the tools.  The applications are generally available for all distributions, we're talking about roughly the same kernel and gnu utilities after all.

If your looking for floating point operations investigate re-purposing video cards.
Here's a teraflop:
http://hothardware.com/News/Asus-Debuts-P6T7-WS-SuperComputer-Motherboard/

Have a look at the new installer, it comes bundled with a modularized kernel.
http://www.plugcomputer.org/plugwiki/index.php/SheevaPlug_Installer

The new installer and mainstream kernel may ease some of the challenges people face testing stuff.  It will certainly make things like NFS4, USB hardware and other items not compiled into the marvel kernel easier to work with. 

I still don't see any reason why you couldn't package up file-system tar-balls, but apt is pretty efficient.  I suspect many of the challenges are related to customizing the kernel for specific situations rather than challenges customizing the file system.

Try removing the verbose flag.  You may be losing an error message in amidst the file listing.

Have a look at:
http://openplug.org/plugforum/index.php?topic=67.msg1452#msg1452

you will need a tftp server, may need to rename one of the files, and the usb detection is flakey. You may need to reset the plug more than once.

I was able to do a USB recovery based on the ubuntu-9.0.5.Release.jffs2  filesystem (file size: 142475264) and the version 1.2 usb recovery u-boot and u-image with an ext2 formatted flash drive.

Two caveats that aren't covered clearly in the documents however.  the reset is critical, I had a number of times when the plug did not detect the USB filesystem until I did another reset.  It also searches for specific filenames on the drive when you type rcvr.  in paritcular uImage.recovery,  a symlink worked.

Where might I find these settings for /etc/exports?  i'm having trouble finding the documentation.  Are these plug computer specific documents or the more generic nfsroot documents?