OZ3.5.1 with GPE
Qtopia removed/X11 replaced
Yep.
blackbox/fluxbox/icewm (1 of them)
Matchbox actually, but from your list you sound like you just want a wm. You/someone could always add it to oe.
filesystem on sd/cf card
This can be done.
writeable root
Yep.
easy usb configuration
What do you mean? Seems easy enough to me.
full sd/cf support
Not sure what you mean, but it certainly works for all of my cards and devices.
sound and midi libraries/drivers (libSDL)
Don't know about this, sorry.
integrated wlan drivers
Yep.
libc2.3.2
Yep.
full compatibility with pdaxrom apps
This is not possible with the 5500 as pdaXrom apps are compiled using ARM5; this is incompatible with the ARM4 SA processor on the 5500. I believe that pdaXrom apps will run on the C machines using GPE, so this is part way there I suppose ;-)
built in ftp/ssh
Not ftp, ssh/scp, you can install ftp from the feed.
minimal apps, nothing comes installed except configuration utilities
What you don't want you can remove - writable root.
Si