So a rigger with an RCC and a bunch of slaved drones jumps in to one of his drones. Can this rigger then use the send message/command multiple drones actions while jumped in?
When you’re jumped into a vehicle or other device, you’re in Virtual Reality mode. (Core 266)
No rule mentions any restriction on Matrix actions as a result of jumping in. Commanding your drones is a Matrix action and you are present in the Matrix. It should pose no difficulty.
Shadowrun 5e CRB p267
Your RCC manages several parallel connections at once, so you can give a command to one, all, or someof your slaved drones with the same Simple Action. This multi-connection also lets you jump from one slaved drone to another without first jumping out of the drone you’re leaving.
Pretty straight forward. Note this relies on the pilot program and autosoft of the drone, you are not remote controlling them. Also jumping in is a complex action so you can't jump in + command drone in the same initiative pass. But by RAW it seems you can give a one phrase command as a free action:
Shadowrun 5e CRB p 164
One short phrase of verbal communication is a Free Action.
Presumablly a drone's pilot program can understand a simple "shoot back" or "follow" command.