Normal children often refuse consent, for injections or dentistry for example, but we take no notice and go ahead anyway.

I believe that the Person Centered Planning literature may have some potential answers for you.  Whereas this process is focused on implementation of intervention programs for people with DD (rather than research), it emphasizes inclusion of the individual with the disability in the decision making process.  Although this is a much more stringent standard than that applied to research studies (especially non-invasive studies), there may be specific methods from that approach that you can use in developing your research protocol - if indeed your goal is to maximize the possibility of true assent in your research study. For example, in PCP one might consult in advance, with a caregiver about techniques that may help maximize attention during a meeting with the individual with DD.  Hope that helps!

Thanks for the feedback! The youth who I work with are most often those with very low cognitive and adaptive functioning, so much of the intervention I am planning, at least initially, is highly dependent on external prompts, but I am working on ways to build choices into the protocols (e.g., with visual supports). I will definitely look into the Person Centered Planning literature!