Yeah, in that case snapd-control would be your only option (you could indeed try to re-pack the greengrass snap, its just a downloadable squashfs file that you can re-assemble with snap pack and add via the --snap option to ubuntu-image , but i doubt any of the interface auto-connections it uses would work then)…
Not that it helps but there was a less strict variant of snapd-control in discussion in the past that i.e. could allow access to config options (but not to start/stop/add/remove snaps), not sure what happened to it though…