I am the product manager for Backbone Issue Sync for JIRA (https://marketplace.atlassian.com/plugins/com.k15t.backbone.backbone-issue-sync/cloud/overview). We are currently in the process of releasing our real Jira Cloud to Jira Cloud Sync.
At Summit, I talked to Dave Meyer, that we are missing a REST API in order to resolve the workflowscheme and all workflows for a given project. I know that there are these internal REST APIs which fulfill our needs:

• JIRA_BASE_URL/rest/projectconfig/1/workflowscheme/PROJECT_KEY
  This returns the workflowscheme for a given project.
• JIRA_BASE_URL/rest/projectconfig/1/workflow?workflowName=WORKFLOW_NAME&projectKey=PROJECT_KEY
  This returns all the details about the workflow with the given name and for the given project.
• JIRA_BASE_URL/rest/workflowDesigner/latest/workflows?workflowName=WORKFLOW_NAME&projectKey=PROJECT_KEY
  This returns all the details about the workflow with the given name and for the given project. I think this one returned more information.

But they are not accessible in the Cloud environment with a JWT token, only with HTTP Basic auth since they are not official.