Feeds Import Alter for Multiple Fields
I ended up solving this myself.  My main question I guess was really about a feeds tamper plugin that can insert multiple values.  Turns out it's pretty easy, just return an array and I think set the 'multi' value on the plugin.  I ended up copying the "explode" plugin and modifying it for my needs.  Here is the result.  It's got some hardcoded values which I'm sure could be coded into the form settings.  I kept it simple for my purposes.

$plugin = array(
    'form' =&gt; 'my_feeds_tamper_image_import_form',
    'callback' =&gt; 'my_feeds_tamper_image_import_callback',
    'validate' =&gt; 'my_feeds_tamper_image_import_validate',
    'name' =&gt; 'My Image Import',
    'multi' =&gt; 'direct',
    'category' =&gt; 'List',
);


function my_feeds_tamper_image_import_form($importer, $element_key, $settings) {
    $form = array();
    return $form;
}

function my_feeds_tamper_image_import_validate(&amp;$settings) {

}

function my_feeds_tamper_image_import_callback($result, $item_key, $element_key, &amp;$field, $settings, $source) {
    $out = array();
    $index = 1;
    $fileExists = true;

    do {
        $filename = "public://import/" . $field . "_" . str_pad($index, 2, "0", STR_PAD_LEFT) . "_lg.jpg";

        if( file_exists($filename) ) {
            $out[] = $filename;
            $index++;
        }
        else {
            $fileExists = false;
        }

    } while($fileExists);

    $field = $out;
}