Since that's my understanding too - I am (slowly) bringing my patches forward to 7.0 - at the moment testing the MC layer changes on powerpc-darwin9 (looking reasonable, as an assembler for GCC, so far). When things are "ready" (at least not broken as far as I can tell) - will push them to my github (iains).

This seems like a reasonable start, and is a step forward.
For the VAArgs, this will be self-consistent, but to be compliant with the system ABI, I think you'll need to account the different padding rules for small aggregates, as per pp14 of "Mac_OS_X_ABI_Function_Calls". Specifically, 1 and 2 byte aggregates are right-aligned and padded on the left, but sizes 3 and above are left-aligned and padded on the right.

(no comments on the actual patch) .. but...
Is the ABI documented somewhere public?
[I have a patch-in-progress that implements binary output for MSP430, it is likely that I'd need to update it to reflect the ABI]

As of now, for targets that do multiple-levels of splitting to legalise variables this patch isn't functional.