There is at least one other way to fly aerobatics within the confines of a federal airway that I know of. I believe it falls under the FAA waiver process. A designated aerobatic
"box" may be established in airspace where it is found not to be an unusual hazard. I did my basic aerobatic training at Santa Paula, CA. (KSZP) The SZP "box" was located 5 NM E. of the airport and right under a Victor airway. It went from 2,500 MSL to 5,000 MSL in an area where the ground level is about 300' elevation. To use the aerobatic box required notifying flight service in advance, who would then issue a NOTAM.
If you look down while flying the Fernando arrival to Burbank and passing over the FIM VOR at 11,000', you can sometimes see an airplane 1 NM below, maneuvering in the box.
This kind of arrangement may be less necessary in the sparsely populated areas of flyover country than it is in more crowded airspace such as that found in or near the Los Angeles or other metropolitan areas. Here, such waiver "boxes" are somewhat more common. One would have to fly a long ways to get clear of airways otherwise.