Another idea, using Blueprints with the same function (StartDash) being called on the owner-client, replicated to run on the server, which then multicasts to all (but only executing on server + other clients / early-out on owner-client):
So basically just pass along that this is the multicast case being called (if the server does it) and if so, exclude the original client. As someone earlier noted, this comes with the disadvantage of unnecessary network traffic (packets to the owner-client which is going to ignore it), but I expect it to be fine if this happens occasionally.
In my case the non-owner clients can handle the entire Dashing/StopDash (timing/states - no movement logic) themselves, so no need for replicating a StopDash (which server would never time correctly, due to lag) and the animation system has all state/timing info it needs.
And yeah, an option "Multicast (except owner-client) would really be convenient.