Using SDCC one can just call or embed the generated C in your version of
char _sdcc_external_startup (void)
This function is called right after setting up the stack and before any other initializations.
Other compilers (like IAR) have a similar C function. Is this one of those areas where Keil is more cumbersome than others? ?

Not really, I have no knowledge that calling a C function at the 'critical place' will be a problem; however. I am not very comfortable running C before the code is totally initialized. Maybe I'm just too 'scared' but "better safe than sorry".

So, for the SILabs (and other with pin/memory/... configuration) I insert the critical initializers at the very start in assembler. Things like UART initialize I do 'at the top of main' in C.

Erik