<?php

add_filter( 'field_editor_text_field_attributes', 'smyles_add_text_field_attr', 10, 4 );

function smyles_add_text_field_attr( $attr, $key, $field, $value ){

	if( $key === 'some_meta_key' ){
		// Add readonly attribute (replacing any others added through this filter)
		$attr = 'readonly="readonly"';
	}

	return $attr;
}