Thinking about it as an alternative to scripted workflows with Rundeck, it seems like the main gap here is getting scripts to run on a specific machine without needing to have set that machine up to run worker processes.
(I could be misunderstanding here, because I haven't spotted anything in the docs that talks about the minimum deployment necessary just to allow a particular machine to run workers.)
Really simple example: there's an administrative command on a mailserver, which can be used to create a new mailbox. I want to make that operation part of some workflow. With Rundeck, I can script this and I just have to make sure Rundeck has ssh credentials onto the mailserver; at execution time it'll deploy the script, substitute in whatever variables, run it and capture the output.
I'm not clear on how I would achieve that with Windmill. Would it involve installing Windmill (or some component of it) on the mailserver, and setting it up as an individual worker group?
And you would do exactly what you suggest in the last paragraph