How do I create a unified list of referenced users in a multi-entry entity-reference field in a multi-entry field collection field?
I guess the solution must be to somehow embed a view to feed this
  view, but I'm not sure how to approach that, perhaps that would
  require an additional module?


You're on the right track, use the Views Field View module to achieve that.



You could also do this programmatically if you have a template file or a preprocess function

Ex:

&lt;?php print views_embed_view('blog_posts','block_1', $contextual_filter_value); ?&gt;




To make it as a list, use CSS:

.view-id-test.view-display-id-block .views-row {
  display: inline-block; /* If it doesn't work try display: inline; */
}

.view-id-test.view-display-id-block .views-row .field-content::after { 
    content: ",";  
}

.view-id-test.view-display-id-block .views-row-last .field-content::after { 
    content: "";  /* or content: normal; */
}


.view-id-test.view-display-id-block being the unique parent classes wrapper as you don't to affect every single view and only a specific view &amp; view display. (Use inspect element with your browser, to find a good parent wrapper.)



If you don't want to use CSS, then you will have to do it programmatically in your view's fields template file.

// load view
$view = views_get_view('view_name');
// set arguments for view
$view-&gt;set_arguments($contextual_filter_value);
// execute view
$view-&gt;execute();
// get all travelers from the result 
foreach ($view-&gt;result as $result) {
   // capture the result of your field in an array. 
   $travelers[] = $result-&gt;field_traveler; 
}
//make array into comma list
$travelers_list = array(implode(", ", $travelers));
// print the list
print $travelers_list;