SYS/BIOS XDCTOOLS locate sections

Ian,
which version of XDCtools are you using? We added some functionality recently that would apply in your case, but you would have to use XDCtools 3.20.04 or newer for that.

The first config parameter that you need is Program.sectionsExclude. It allows you to remove sections from automatic allocation. This is how you would specify that you don't want XDCtools to allocate .const, .far and .fardata automatically:
Program.sectionsExclude = "^\.const|^\.far|^\.fardata";

Then, you need to generate your allocation for these sections. The easiest way to do it is to use Program.sectionsTemplate. This config parameter specifies a template that replaces the automatically generated directive SECTIONS with your own content:
Program.sectionsTemplate = "../mySections.xdt";

If you start the path to the template file with "../", then you can keep the template in the same directory with your config script. I am assuming you work in CCS, where your working directory is one level below where your config script is. If your setup is different, and you get an error message that the template file cannot be found, you'll have to adjust that relative path.
Finally, you need to create the template file. It uses XDCtools-specific template syntax, but your goal is simple enough that it requires only a couple of lines of code:

    GROUP: load > 0x80000400
    {
        .const:
        .far:
        .fardata:
    }
`$args[1]`

The first couple of lines contain your allocation for .const, .far and .fardata. The last line add the allocation for the rest of the sections. Check the linked docs for Program.sectionsTemplate for more detailed explanation how it works.