Yes, that you can do, I thought you wanted to add more settings. Fields already have a readonly property, it’s just not editable when creating fields. You can set the value of this property using an acf/load_field filter http://www.advancedcustomfields.com/resources/acfload_field/
In the filter if you do
echo '<pre>'; print_r($field); echo '</pre>';
all of the field’s settings will be listed. Any value of the field can be edited at this point with a few exceptions like the field name, field key, field type. There may be others that you shouldn’t change, I don’t know them all off the top of my head.
If you want to process shortcodes for a field or run some other functions on the value of the field then you use the acf/format_value filter http://www.advancedcustomfields.com/resources/acfformat_value/, an example of running shortcodes for all textarea fields is given on that page.
a note here, you can also use the $field_name and $field_key hooks in ACF5, these are not documented.
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