Using Wheezy there is no problem with ssh daemon: it just works.
So far I have added my own extra chroot to the SD, which I prepared according to the instructions for booting the standard wifi image from SD.
I liked more to mount the custom ext3 image from the SSD as loop file through losetup.
However it should be possible to create a self contained image and let the onboard bootloader to start that one only.
Copying the extra kernel modules (if any) from the default image to the custom one ought to be enough.
I'll post my scripts once I get it to work.