I have confirmed that all 5.00 custom XMB waves work for 6.20- 6.39. You have to replace system_plugin_bg.rco inside flash0 to use (if you have the balls, you can have custom waves on a PSP 3000 or PSP Go).
WARNING- Due to this is meant for firmwares before 5.00, This will break the functionality of the original wave (wich your PSP will freeze and crash if tried to applied, but not bricked.). If you are not afraid to give up the original wave for a modded classic, this method will work (just don't try to apply the original wave and you will be fine).
Here are some pictures of proof of my PSP (sorry for it being blurry. It was taken from my iPod) (PSP 1001- 6.20 PRO B9) (I tried to rotate the pictures but they refused)
Extras + Digital Comics
6.20 PRO B9 Permahack