The plugin you found doesn't have a 1:1 ratio between Heroku instance with build jobs since it takes the job name as parameter, so you should be able to run any number of jobs from a single Heroku instance.

As for passing a space in a job parameter, it depends on how the arguments are split by [this line](https://github.com/joshdholtz/jenkins-slack-command/blob/master/web.rb#L29), I don't know enough ruby to make sure that it would avoid breaking quotes. Another way could possibly to use the HTML spacing character `%20` and see if it works.

Personally I would try to use the plugin to run the correct job with the associated parameters directly instead of going through filtering jobs.