I would like to disable the default "Upload" source in favour of using the upload option within IMCE

This isn't planned because FileField Sources is heavily dependent upon the existing upload widget (even though it may not seem like it). Additionally uploading through IMCE is not supported at all (you may notice you can only browse when using it with FileField), because IMCE does not properly manage its file references. Generally speaking it's actually a very dangerous thing to have installed, because users can delete files willynilly regardless of if they are in use or not.

What about an option to default it to an alternative source.

For Example I may want to encourage users to use IMCE by having the the default option.

put this code into your theme css. it will hide the default upload and show the browse link (to imce) by default.

#edit-field-teaserimage-0-upload-wrapper .filefield-upload, #edit-field-teaserimage-0-upload-wrapper .description, .filefield-sources-list{
display:none !important;
}

.filefield-source, .filefield-source-imce{
display:block !important;
}

Note that with the approach, you still can't upload through IMCE, since uploading is disabled when used as a file browser for FileField Sources.

But you can enable it by editing this function at the end of imce.inc file.

function filefield_source_imce_custom_process(&$imce) {
// Disable uploading, thumbnails, delete, and other unnecessary features.
$imce['perm']['subnav'] = 0;
$imce['perm']['upload'] = TRUE;
$imce['perm']['thumb'] = 0;
$imce['perm']['delete'] = TRUE;
$imce['perm']['resize'] = 0;
}

Also see #879902: Disable upload widget for special cases