The idea of differentiation is about fitting the learning to the most effective way the student will learn. Whether that means something as simple as giving students visual representations to help visual learners to offering technology to help students with hearing problems read or see the words instead of hearing them, and many other possibilities.
When designing an online program, it depends on the program as to how you would adapt the vision of the course. For example, if you were designing a course for the broad internet, like Edx, then it should be assumed that all possible student disabilities will be represented and students with them will want to partake in the course. In fact, it is the law to consider such a thing (Dept. of Education, 2013), at least if you are getting federal funding. I also think it is the moral goal of public education to educate everyone, not just "normal" students, whatever that might mean. However, if you were designing a small scale, blended course, there might not be enough funds to offer all possible solutions right away, so there might need to be much more awareness by the teaching staff of what disabilities the course actually needs to attend to based on the students in the course. A blended course would also afford the opportunity to see and meet the students in question and assess their needs more accurately.
So, how would I adjust a fully online course? I can create a "multi-modal learning experience so students can access course content in whatever way works best for them" (Gomez, 2013). If I can imagine how the course will be for the student, I can offer alternatives in as many ways as I can as a designer, so the students will be able to adjust the learning for themselves.
Teachers can collaborate on each other's course design by role-playing a student with a particular need before rolling out the course, or use personas (Gomez, 2013). Think of what the student might need for the course and test the course to see if it meets the challenges. Or you can use real examples to shape your design, like Shelesha Taylor who has a degenerative eye problem and needs software to magnify the text on the screen (Haynie, 2014). Some of her required readings do not come in the right format to use the software, so she has to work harder to locate copies not offered by the professor. Using UDL, the professor could have offered an audio version of the text or alternative that would have suited the student's needs.
The challenges are trying to predict the unpredictable. Finding problems before they are problems can be a difficult task, but using UDL and offering alternatives will reduce the later challenges because students will have the ability to choose their method of learning a little bit more than normal. Social stigma is another challenge. Many students will not admit they have a learning disability and they might come into the course undiagnosed, so helping a student that does not want help or think they need help can be challenging. However, I prefer to think of disabilities as part of the spectrum of learning and having certain "special" help offered to all students reducing the stigma of using that help. If no one is there, like an online class, the students will feel more comfortable to try something like a magnified screen, for example, but if it is clearly offered to all students, then it will seem less like a special need, and more like a regular tool for learning as well.
I think the biggest challenge will be assessing disabilities over a fully online course with no prior knowledge of the student's abilities or learning styles. Making sure there is data tracking of how the student chooses to view the course (which modal they choose, for example) and paying attention to their work, it can be done.
References
Department of Education. (2013, December). Protecting students with disabilities. Office for Civil Rights. Retrieved from http://www2.ed.gov/about/offices/list/ocr/504faq.html
Gomez, C. (2013, October). Help ensure online course content is accessible to students with disabilities. Disablity Compliance for Higher Education.Retrieved from http://www.disabilitycomplianceforhighereducation.com/Article-Detail/help-ensure-online-course-content-is-accessible-to-students-with-disabilities.aspx
Haynie, D. (2014, April). Students with disabilities meet challenges in online courses. U.S. News & World Report: Education. Retrieved from http://www.usnews.com/education/online-education/articles/2014/04/04/tips-for-online-students-with-disabilities