> I [...] find it hard to believe that there would not be a significant amount
> of platform specific code that must be written for each architecture.

I fail to see a reason why there should be *any* code specific to any CPU platform or CPU architecture in the LibreOffice C++ source code.
Even if there should be some, say, SIMD-optimized (and thus CPU-specific) routines in the source code, then they are already there in the existing PPC version as well (with AltiVec code instead of AVX/SSE code), or there's a generic scalar code path as fallback used in the existing PPC version. Either way, there's no need for any "PPC specific code that was created during the porting process to MorphOS3.x (and/or AmigaOS4.x)".

> Specially for a project as large as this one being proposed

I fail to see how larger C++ source code would require larger amount of code specific to CPU platform or CPU architecture.

> I don't know what version of MUI AROS can use, or if it is used at all

http://en.wikipedia.org/wiki/Zune_(widget_toolkit)

> my limited understanding of AmigaOS4.x and MorphOS3.x suggest that quite a bit
> of coding for MUI4 on MorphOS3.x, and ReAction for AmigaOS4.x, would make the
> platform differences between the two binaries a significant amount of coding,
> but a small percentage of the total coding for each project.

Rewriting the standard LibreOffice GUI code into GUI code for AmigaOS/MorphOS/AROS will be the biggest part of the total work to do. And yes, using different GUI toolkits for the ports to the various AmigaOS-like systems would multiply this amount of work.
Nevertheless, this matter of GUI-specific code has nothing to do with the matter of CPU-specific code. The current LibreOffice GUI source code looks the same for the x86(-64) versions and the PPC version of LibreOffice. Likewise, MUI/Zune source code looks the same on x86 and on PPC. That's why once a MUI/Zune GUI for LibreOffice exists, creating binaries for AROS/x86 and AROS/PPC (and AROS on any other CPU ISAs) will be just a matter of switching the compiler to the respective target CPU.