As Othesus said, it's not possible to do it like that, there is no known way to access that data from within the UI, in particular since they introduced the IconBank for showing hotbar buttons, and I'd wager there is no way at all to do it.
There are other ways which are a bit more limited than what you try to do, but you may want to give them a try:
Since class specific buttons on group and raid windows have been requested a number of times for my UI as well, I have on my list the idea to provide one or more custom buttons for these windows which the user can configure dynamically using a custom window having radio buttons. The group/raid window custom buttons would press the selected radio button on the custom window. You would have preconfigured the radio buttons with any number of commonly used spells you would cast on group members (on top of the list necro hearts or conjuror shards
).
Another idea I have been toying with is to use class auto-detection code and automatically select a preconfigured action for that class to be used when clicking the custom button (again with the example above, necromancers would get the heart button and conjurors the shard button).
Feel free to use any of those ideas.