How can an entity reference field be displayed as a select list that shows the selected entity?
You have 2 relatively simple options without creating your own module and field display code (in a module).


use Views to generate the select lists you want. You can do this for example with a ctools dropdown or jumpto displays. While you're using a view you can always set Views to cache the results of your view so its not cumbersome after the 1st page load for all other visitors. You can even define views and attach them to your node if you want with field_views_field module.
Use Display Suite on your site and create a Custom Field in DS for any fields you want custom output for, like your select list. Then in the PHP area of the custom field you can access the currently loaded $entity fields and generate whatever HTML you want for your field output. You would then hide the default field display of your field, and show your custom DS created field for your content type.


I'm about to do (2) above with a slideshow content type where per-slide the content editor can select the text foreground and background color using the field_color module which would best match an uploaded image to the Node. Then in DS I will make a custom "slide_text" dyanamic field that examines the color values and basically outputs:

&lt;?php
// This is pseudocode and untested.
$wrapper = entity_metadata_wrapper('node' $entity);
$text_color = $wrapper-&gt;field_foreground_color-&gt;value();
$bg_color = $wrapper-&gt;field_background_color-&gt;value();
$text = $wrapper-&gt;field_slide_text-&gt;value();
echo &lt;&lt;&lt;EOFHTML
&lt;span class="ds-slide-content-field"
      style="color: $text_color;
             background-color: $bg_color"&gt;
    $text
&lt;/span"&gt;
EOFHTML;
?&gt;