Is is possible to anonymously encourage one's US university to make content accessible (e.g., as per the ADA)?
The appropriate place to raise this would be through the Disabled Student Services at your university. Even though you're taking the course as a MOOC, the DSS office still has authority over the course (unless it's being run through a subsidiary or joint-venture similar to Ed-X) as well as the responsibility to make it accessible. C.F. Harvard and Ed-X accessibility settlement. 

If DSS does not respond, then the next place to go would be the ADA Compliance Officer on campus. If the university is receiving federal funds, they should have someone designated as such (and this person might share multiple roles, such as the Title IX Compliance Officer, etc.).

If that fails, then the university ombudsperson.

If all university channels fail, then you contact your State office of civil rights or  the Federal Office for Civil Rights (OCR). It's unknown whether the Federal OCR will be proactive and responsive during the current regime, but technically they are where the buck stops right before you go with a full blown ADA complaint.

Also, you might also pursue hiring private legal counsel, especially if you absolutely want to remain anonymous during the initial complaint phase. Ask your local disability center for independent living for referrals to local attorneys who take on ADA/504 complaints against universities (note this is very different from run of the mill ADA cases against private businesses) or consult with a national firm such as Disability Rights Advocates, CREEC Law, or DREDF, etc.