Is it possible to target a fieldtype by it's Type name, rather than the handle name?
I am not sure if I am understanding your question correctly, so here is my understanding before I give my solution:

You want to be able to access a field on an asset by it's field type rather than it's handle.

Given this understanding, here is my solution:

I have not been able to accomplish this using Craft relations. However, you can loop through the fields attached to the asset, and check their type. It is a little bit confusing as Craft stores the data for fields and field layouts across several tables. 

An element has a field layout (FieldLayoutModel), and the layout then has fields (FieldLayoutFieldModels). Then each of those FieldLayoutFieldModels has fields (FieldModel) which are the actual fields we are looking for. Putting this together, we can loop through and check the field type like so:

    // get the fieldLayoutModel
    $fieldLayoutModel = $image-&gt;fieldLayout;

    // get array of FieldLayoutFieldModel
    $fieldLayoutFields = $fieldLayoutModel-&gt;fields;

    foreach($fieldLayoutFields as $fieldLayoutField){

        // get the field associated with the layout
        $field = $fieldLayoutField-&gt;field;
        if($field-&gt;type == 'myFieldType'){
            // do whatever
            break;
        }
    }


Hopefully this is what you were looking for!