1. There is no API to open the rpm and find the .spec file inside it. So yum+mock+koji+whatever would need to start running rpm cli commands to do this kind of thing.

2. The assumed model everywhere is that the srpm headers aren't arch specific (and don't need to be interpreted). Eg. rules about how we can't use %{_isa} in buildrequires etc. ... fixing this is probably "not easy".

Please discuss it first with Panu Matilainen.  I have filed this Bug as I was told so in Bug 664427 Comment 14.

I do not mind in which component how it gets fixed, I am just a developer and I need rpms working.

Although the following page isn't an official part of the FPG yet, the draft specifies the problematic thing:

http://fedoraproject.org/wiki/PackagingDrafts/ArchSpecificRequires

in the following section:

In most cases, adding an arch specific dependency unnecessarily will not cause problems. There are a few, however, that MUST NOT be made arch specific:

    * A package has a Build-Requires on a specific arch. library (because rpmbuild evaluates the %{_isa} at .src.rpm buildtime, and not at .src.rpm => .arch.rpm build time).
    * Specifying an arch specific dependency on a package that is noarch. 

and notice the strong wording here with a "MUST NOT" use %{_isa} in a BuildRequires.

Thanks & regards, Phil

The page is just WONTFIX of this Bug by other words.
I have added disagreement there to the last "Question" item.

 What Panu said is:

What we can do to migitate the issue is backporting the necessary python
bindings bits that makes it possible to use yum-builddep on specs directly.
Devel ack for *that*, and that alone.

...and then in https://bugzilla.redhat.com/show_bug.cgi?id=664427#c22 he said he's fixed it by changing rpm so that:

[root@localhost pmatilai]# yum-builddep -q libselinux.spec 
Getting requirements for libselinux.spec
 --> Already installed : python-devel-2.6.6-29.el6.x86_64

...so AIUI there's nothing to do in yum-utils. Note that this only works directly on specfiles, if you do "yum-builddep libselinux" it's not going to work (and can't).

This should work for:
$ yum-builddep -q libselinux-X-Y.src.rpm

And it can work.  As one can do:
$ rpm2cpio libselinux-X-Y.src.rpm | cpio -i libselinux.spec
$ yum-builddep -q libselinux.spec

and if a user can do it then yum-builddep can also do it.

This request was evaluated by Red Hat Product Management for
inclusion in the current release of Red Hat Enterprise Linux.
Because the affected component is not scheduled to be updated
in the current release, Red Hat is unable to address this
request at this time.

Red Hat invites you to ask your support representative to
propose this request, if appropriate, in the next release of
Red Hat Enterprise Linux.

> and if a user can do it then yum-builddep can also do it.

While that's technically true, it's often not that easy to just do it.

Given how late it is in RHEL-6 I'm going to close this one. You can open a bug against RHEL-7, to add this feature but it isn't upstream yet and without a patch I wouldn't expect this to land in RHEL-7.