Flash builder 4 - change output filename using external build-config.xml (not Ant)

Question

I'm trying to change the output filename with a config file loaded via the compiler option -load-config. It looks like this in my compiler arguments:

-load-config+=build-config.xml.

I've tried the following:

<flex-config>
    <o>absolute/path/to/filename</o>
</flex-config>

and

<flex-config>
    <output>absolute/path/to/filename</output>
</flex-config>

and

<flex-config>
    <compiler>
        <o>absolute/path/to/filename</o>
    </compiler>
</flex-config>

and

<flex-config>
    <compiler>
        <output>absolute/path/to/filename</output>
    </compiler>
</flex-config>

but none have worked. I'm on a PC using Flash Builder 4. Has anyone else done this? Also, ideally, I want to use a relative path instead of absolute. I can't get this to work either, even if I do so in the "additional compiler arguments" field of the Project configuration.

Thanks in advance!