It works on the ZX300 because I actually dumped my device configuration and sent it to the developer so he could add support for it.
You need to run the following commands and send the sonynwz_result.txt to the developer (see help_us in the rockbox page)
scsitool-nwz-v12 X: get_dhp >> sonynwz_result.txt
scsitool-nwz-v12 X: get_dnk_nvp_multi kas shp sps fmp pcd mid ctr fpi fur fvi ins lyr nvr prk rbt she sku ums ups clv pwd clg >> sonynwz_result.txt
Replace X by your walkman drive letter.
Make sure you use the latest version.
I sent the device information to their email address. Hopefully I can get the player changed and running soon! I took out the blah_blah from their email address. Is that right? Sorry, I’m an absolute noob. I had to dig through this forum when I found out my device was for Japan only.