You are correct that LTI 1.1 only supports passing back a numeric grade between 0 and 1. However, with LTI 2 now supported in Moodle 2.8, the result service should become available soon which supports the addition of a comment field which may meet your needs. In addition new services for managing line items and results have just been released as a public draft (see http;//www.imsglobal.org/lti) and also support the comment option and I would expect to be made available within the Moodle module. If this is not sufficient for your needs, you could also consider writing a custom (bespoke) service which is secured using the LTI credentials. In that way the same code at your end could work with other platforms which implement the same service.
by Stephen Vickers.