Where can I find a list, or some kind of summary, that details what specific modifications I can ask the school to make for my child under 504 regulations? I've had counselors say, "Yes, we can do this," or "No, we can't do that," but I just have to take their word for it because I can't find anything in writing.
You can't find a list because specific modifications are not in the law. Section 504 of the Rehabilitation Act of 1973 guarantees that all agencies that receive federal financial assistance provide "access" to individuals with disabilities. Historically, this law has been used to require public agencies (including schools) to install wheelchair accessible ramps and restrooms, interpreters at meetings, etc. It also requires school districts to guarantee that children with disabilities are provided "access" to educational programs and services by providing note takers, readers, interpreters, technology, etc.
Although the law does not list specific accommodations/modifications, you can find that information in other resources. For example, in his book, Problem Solver Guide for Students with ADHD, Harvey C. Parker lists three pages of possible accommodations parents might ask for. You can also have a look at Lawrence M. Siegel's The Complete IEP Guide: How to Advocate for Your Special Ed Child. Finally, check out the website of Children and Adults with Attention Deficit Disorders for tips on obtaining modifications or accommodations in your child's program under Section 504.