There are two entirely different kinds of attics being discussed here. One is a maze of trusses. The other is a large lofted space with a floor and headroom and easy access from a stairwell. I'd say flat NO to putting a panel in most houses in my area, but quite a few houses, especially in new england, have steep pitched or simply high-peak roofs with no trusswork and not only have quite useable space, but often full stairs up to them.
A friend of mine's attic is so large, he built a full apartment in it with stairs and everything- all finished, heated, etc. He put a full sized door opening into the attic-attic portion to use for storage and access to the HVAC equipment. In this case, the attic would easily meet all accessibility requirements, and the only problem problem I'd see with putting an electrical panel there is how hot it would get in the summer.
