1. A method, comprising: in response to an instance configuration request from a client, receiving, from a selected control server of one or more control servers designated to manage remote configuration of resource instances, a sequence of one or more commands formatted according to a particular command protocol for instance configuration, wherein said receiving is performed at a selected instance host of a plurality of instance hosts designated to implement resource instances of a network-accessible service; instantiating, at the selected instance host in response to receiving the sequence of one or more commands, a remote command executor; initiating, by the remote command executor at the selected instance host, one or more configuration operations corresponding to the sequence of one or more commands; providing results of the one or more configuration operations to the selected control server; terminating the remote command executor; and providing a response to the instance configuration request, based at least in part on the results, to the client.