In blizzard there is a macro for example
#showtooltip
/cast [@player] Death and Decay
This allows the spell to be placed directly on the feet of the player, it is mandatory rotation for 2 one hand rotation, in which a lot players will play in upcoming Onyxia progress.
I think it can be done and i see no reason to not have this, this could be useful for other classes too i guess. Devs are smart and can make it work easier if the staff of Warmane approves it.
as far as i know, the onyxia realm send custom data, so if the client send this data to the server, the server can register it and just cast the spell on the current position of the player, i mean sound easier i know is way more complex but the general idea is this
why not just put it down manually? a gcd is plenty of time to do so, and it's also better for moving targets and multiple targets to just place it down manually on the best spot. I have played quite a bit of dualwield uh, and dont see the point, let alone necessity of such a macro command