Content referencing the referenced content I've been searching and cannot find a way to achieve this: display content referencing a content that is referenced by the content being displayed.

It may be somewhat confusing but I'll do my best to be clear:

I have two content types: 'Company' and 'Service'
As you can imagine a Company provides many Services, so there is an Entity Reference field in Services that points to the Company providing it.

That's it:  Service content points to Company content in a many to one relation, many services point to one company (the company that provides them)

When I display a Service, I also display the company that provides it through the Entity Reference field.

Now the tricky part is:
When displaying the page for a Service, I'd like to also show (e.g. in a sidebar) the other services provided by the Company that the displayed Service is pointing to. 

I'm using Panels and Views on Drupal 7.

Please note: this is not a simple 'related content block', that's what I am using to display the company providing the content or when displaying the Company content
I don't see any problem here. or any difficulties to do this.. You only need a block with services of certain company, so you can create a views block , pass company id as argument (by contexual filter), and exclude current service from it.. But, I don't think that simply assigning it to region can help you, because you will need to fetch company id from service page and pass it to view block. I think the solution will be to implement hook_block_info and hook_block_view hooks to create another block and fetch company id from service page and display it.. 

example:

function MYMODULE_block_view($delta = '') {
  $block = array();

  switch ($delta) {
    case 'company_service':
        // there is multiple ways to get field value from entity. In this example I am it getting with entity_metadata_wrapper function from Entity API:

        $service_node = menu_get_object();
        $service_wrapper = entity_metadata_wrapper('node', $service_node);

        $company_id = $service_wrapper-&gt;field_company-&gt;value(); // I don't know the structure of your entity fields, that's why we assume that $service_wrapper-&gt;field_company-&gt;value() will get company_id..

        $block['subject'] = t('Company services');
        $block['content'] = views_embed_view(VIEW_NAME, DISPLAY, $company_id);
      break;
  }
  return $block;
}