w.r.t. the availability of OSS compilers for Embedded PPC:
I worked on several very large and complicated embedded PPC projects using UBoot and GCC from DENX (http://www.denx.de/en/Software/WebHome).
As long as you have a stage-0 bootloader and a serial port, the rest is very workable with OSS tools.
In another life, I also wrote all the multistage bootloaders for an ARM926EJS based part from stage-0 to up to Montavista Linux entirely with GCC and Binutils.
Using LD to generate embedded software images can be tricky b/c the linker script language is complicated, but it is very doable once you get a feel for it.
Having a spectrum digital JTAG pod to debug the stage-0 bootloader was necessary, but once you get the serial port working, the rest is usually gravy. Those pods were about $1500 with software. These days, tin can tools do a decent job for some parts.
I also disagree with notion that a RaspPI wouldn't survive very long as a limiting factor. While the whole board isn't rated for industrial temp range, the BCM2836 is supposedly -40C to 85C. It's not a huge stretch to believe that a similarly priced board (even 2x or 3x the cost) could be completely wide-temp rated.
What would be of concern is possible peripherals that are sometimes used in automotive ECUs, such as advanced timing modules, that are used to provide very precise hardware based timing of things such as injector control and (in spark ignition applications) spark plug firing not being available in generally available consumer parts.
Now, if you have regulatory compliance requiring signed images, crypto, etc. Then it goes more complicated, but not unworkable. There are a number of vendors that offer hardware crypto chips commercially for such applications to assist. These are not prohibitively expensive either.(http://www.atmel.com/products/security-ics/cryptoauthentication/)