I'm not sure about a measure of "empowerment" but Mike Wehmeyer developed a "self-determination" scale for persons with intellectual disability - called the ARC Self-Determination Scale: http://www.thearc.org/document.doc?id=3670.  It might be well worth a look at. 

http://ngsd.org/news/arcs-self-determination-scale

I'm not sure that measurement is necessarily going to provide much information. However, student self evaluation of learning and personal involvement in target setting has proven successful with learners with intellectual difficulties in the UK and Ireland. My own research in this area, much of it conducted with my colleague Michael Shevlin from Trinity College Dublin, suggests that a respect for student abilities, rather than a focus upon deficit models can encourage successful self evaluation. The following may be of some interest:

Rose, R, & Shevlin, M. (2004) Encouraging voices: listening to young people who have been marginalised. Support for Learning 19 (4) 155 – 161

Rose, R. (2003) Pupil participation in target setting procedures. TOPIC: NFER review of current research  29 (Spring) NFER.

Shevlin, M, &  Rose, R. (2003) Encouraging Voices: Respecting the Insights of Young People who have been Marginalised. Dublin: NDA

Shevlin, M, & Rose, R. (2008) Pupils as partners in education decision making: responding to the legislation in England and Ireland. European Journal of Special Needs Education. 23 (4) 423 – 430

Rose, R, & Shevlin, M. (2010) Count Me In. Ideas for Actively Engaging Students in Inclusive Classrooms. London: Jessica Kingsley

I really agree with Marc Tasse. The ARC scale may be good for your interest.