Thorsten,
I am not aware of a strict 4MB kernel limit - the kernel needs to fit into ST-RAM entirely. ST-RAM may be as large as 14 MB.There is one, but that seems to be related to, at least, page table setup in early kernel space (but my attempts at patching that didn't work either, so there is more).
I suppose you changed the mapping in both head.S and later mm init code? (INIT_MAPPED_SIZE in mm/motorola.c) Can't see anything limiting the initial mapping to 4 MB at a quick glance. The only complication might be that the memchunk size isn't known at that stage (not certain to what extent bootinfo is parsed in head,S). 4 MB is a safe bet.
On the other hand - as long as paging_init and code calling it are in the initially mapped space, it should not matter.
The initrd can be loaded anywhere - if the bootstrap supports loading the initrd to TT-RAM you should be fine.OK. We need "lots of" TT-RAM already anyway, so... wfm.
You're right there. Good to hear the initrd load to TT-RAM works.
Cheers, Michael