Eventually, after a lot of hacking, I ended up with a kernel and filesystem that booted into the SIP-supporting hacked image. The nasty thing about it was that it took a Linux-machine, a WiFi-connection, an USB-connection and a serial connection to do it that way. I decided to look into the official SMC flashing tool. While the WSKP100 flasher seemed to work with unusable flash-images, probably to protect the Skype binaries, the SMCWSP100 flasher worked with plain old unencrypted binaries. Because I fiddled with the partitions the flash is divided in, I had to adjust the offset various files were written to. Luckily, these were available in plain old ascii in the flasher file. After a bit of testing, I had a tool to flash phones to my hacked version of the firmware.
You're probably itching to try the tool and firmware yourself now, so if you want to download it, it's here (15MB). Some people have reported that the WLAN-chip inside the phone gets a MAC of FF:FF:FF:FF:FF:FF; unfortunately I haven't found a fix for that yet. If you can tell me where the MAC on these phones is stored, please do. The source of my version of the kernel (Warning: quick hack!) is downloadable too (44MB).
I still will try and develop my own software for this nice device, when I have some time on my hands. For now, this hacked-together firmware is a nice stopgap, and a cost-effective at it: I've seen the official SIP-version of this phone for twice the price I bought this one for.