I believe a "XBE_MEDIA_HDD" flagged dashupdate.xbe does exist.
If you ever noticed, if you:
1: upgrade your dashboard to the latest "Live" enabled dashboard
2: delete all the contents of your C drive
3: downgrade to lets say 4920
4: upgrade to the latest "Live" dashboard again
It does
not "re-download" any of the new dashboard files, it starts the update from the hard disk itself (I left the network cable in only long enough for it to find out it needed a update, then unplugged). Hopefully the dashupdate.xbe (if that is what is used) might be stored in the fatx or some other drive I am unaware of.
I hope this has been helpful, I might be completely off

-devz3ro
http://sh0x.tk/