Use patch directly then. E.g.
    %patch1 -p1 -F1
should be written as
    patch -p1 -F1 < %{PATCH1}

It's just a steenking script ...

That makes for awfully cluttered spec files when you have a lot of patches applied

So write a script that applies patches, add to the Source: list,
and run that script like
   Source123: mypatchapplier
...
    %{SOURCE123}

It's just a steenking script ...

Pardon
    sh %{SOURCE123}

And its's still a steenking script, just withou execute bits ;-)

That's a lot of hoop jumping to work around something that should be a trivial
change to rpmbuild.

Changing the source code to add an option is trivial.

Introducing an incompatibility in builds -- particularly for kernel --
forces a non-trivial deployment of rpm.

"Clutter" (at least by metric no. of bytes) can be addressed by writing a macro
something like
    %define mpF  patch -p1 -F1
to be used as
    %mpF < %PATCH1
to remove "clutter".

I don't believe that adding -F1 to %patch in rpmbuild is a sound idea.