Sharp released package for installing flash for the Ubuntu/ARM based NetWalkers. It might work on ubuntu/debian. JKKMobile posted a copy of the installer here http://www2.jkkmobile.com/FlashLite3.1_Firefox_plugin.tar.gz
I hosed my ZUbuntu install a couple days ago in a misguided Maemo/Mer experiment, so it will be awhile before I can try this myself. I looked at the tar file and extracted the installer. Its pretty simple, the tar has couple .so files an a installer.sh that just copies them to the correct directories. It may work. Anyone want to give it a try?
(edited title to make it clear that we are talking about using NetWalker's flash package on the Zaurus, sorry)