Since my original report on the 'Sandisk - Installation/Removal' subforum was delted I'll report it here, where it should be appropriate:
I tried installing rockbox on my c240v2 according to the instructions in the "Installation for Unusable (other AMS Sansas)" section of http://www.rockbox.org/wiki/SansaAMS
I ran into the "C200V2 bootloader installed but no effect" issue, but was able to work around it by patching dualboot.S, apparently the USB pin check is borked for my device and always branches to the original firmware even if the device is not plugged into USB.
See also http://forums.rockbox.org/index.php?topic=23263.msg162027#msg162027
After flashing the firmware with my updated dualboot, it now tries to boot rockbox, but apparently fails.
The exact behaviour differs depending on whether or not .rockbox/rockbox.sansa is there, so I'm assuming that the second-stage bootloader works so far.
However as far as I understand the second stage is supposed to turn on the lcd, show a splash and report an error there if rockbox.sansa is missing. My lcd does not turn on though.
What I do see:
If .rockbox/rockbox.sansa is not there:
Middle button light turns on for some time, device powers off.
If .rockbox/rockbox.sansa is there:
Middle button light turns on for some shorter time, turns off, both middle button and menu turn on
for some longer time, both turn off, pattern repeats.
If I boot while holding right it boots the original firmware just fine, so my patched dualboot works for me.
I tried building a newer second stage bootloader from svn than the Nov. 21st 2009 version at http://download.rockbox.org/bootloader/sandisk-sansa/c200v2/bootloader-c200v2.sansa
But the behaviour stayed the same after flashing that one.
Hope that helps.
--- dualboot.S (revision 24590)
+++ dualboot.S (working copy)
@@ -93,12 +93,14 @@
#ifdef USB_PIN /* TODO : remove this check when we'll have an USB driver */
ldr r0, =GPIOA
mov r1, #0
str r1, [r0, #0x400]
ldr r1, [r0, #(4*(1<<USB_PIN))]
cmp r1, #0
/* Here are model specific tests, for dual boot without a computer */