This works quite well, thanks @cmatsuoka !
An improvement that would make this nicer to use is to allow --arch=arm64 for example even when the architectures key is specified. In this case I would expect snapcraft to ensure that the specified architectures are listed in architectures as build-on, and then only run that build. As it is, I only need to build one specific architecture, arm64, remotely, but I currently need to launch unnecessary architectures (i.e. amd64) on launchpad.