unknown.obvious, I think we need to develop a naming convention to add your kernels support to the script until those get published somewhere more "official"
Can you make the archives unpack to folders with names starting "custom_kernel*", "custom_n900_kernel*" or whatever you like?

UPDATE:
Looks like Tom Gall wanted to make a hardware pack for N900. I'll try to contact him.
https://wiki.linaro.org/TomGall/LinaroN900