Thanks Nate! Yeah, so far I'm using the sync config to select all keys by choosing the respective white key in the dopesheet. However, another interaction inconvenience is the visibility of keys in the graph. In spine, when you select a bone, you see the keys of that bone (in the dopesheet and graph). However, when you deselect the bone (by pressing ESC, for example), you see all the keys in the dopesheet, but it doesn't happen in the graph. In the graph, you still see only the last selected bone, unless you manually activate the visibility of all bones and their keys.
In the other hand, regarding modifier keys, you are completely right. I think my main hassle is the described above, and wouldn't need a modifier for all keys in a specific frame. Nonetheless, it would be great to have custom modifier keys for selected groups of bones. This more or less already happens with the white keys for saved selections (CTRL+#), but this could be nice for complex projects. Especially those involving several skeletons for instance (as we can't, currently, collapse the timelines of entire skeletons)