Thank you.
That's an idea worth exploring even if I am not sure it is always called upon... I will certainly try it.
Yet, further reflection made me conclude that if that does not work I could just create a "slash command" to replace /cast in the macros... That would certainly work (although it will require people to rewrite their macros, which I wanted to avoid)
Thank you again, I will try your idea first...
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Addition (30 minutes later)
Nah.. CastSpell(Id, ...) is only invoked when you click on a spell directly in the spell book.
Turned out it doesn't work...