How to build noarch package on only certain architectures?

[Eric Smith]

I'm trying to build Clapham for EPEL6.  Clapham is a noarch package, but 
it depends on Batik, which is ExclusiveArch: i686 x86_64.  When I 
submitted a build with koji, it failed to build for ppc and ppc64 due to 
the missing dependency.

I tried adding the same ExclusiveArch to Clapham.  That causes errors 
from rpm and koji.

Now I've tried adding ExcludeArch: ppc ppc64 s390 s390x.  Koji still 
tries to build it for ppc64, and it still fails due to the missing 
dependency.

Is there some other way to add a noarch package that doesn't build on 
some architectures?

Thanks,
Eric