I ended up with a LESS plugin. less-lib.js:

registerPlugin(
    {
        install: function (less, pluginManager, functions)
        {
            functions.add('rp', function (rpx)
            {
                return new tree.Dimension(rpx.value / 16, 'rem');
            });
        }
    }
)

Using:

@plugin "less-lib";
.my-class
{
    width: rp(20);
}