Response from developer relations @ Atlassian concerning how to get a plugin available on JIRA OnDemand.
"We are no longer accepting "Plugins v2" plugins from third party vendors in OnDemand. The way forward is to write an add-on using Atlassian Connect. The Atlassian Connect project is still in development and now is a good time to get involved if you want to ensure your add-on will be compatible.
If you want to learn more or ask questions, you can join our google group: https://groups.google.com/forum/?fromgroups#!forum/atlassian-connect-dev"
That means, that JIRA Syntax Highlighter cannot be easily transfered to OnDemand. It needs to be re-developed as a Atlassian Connect plugin.
Atlassian told me that they will not accept Plugin v2 add-ons anymore and not even JIRA Syntax Highlighter as one last plugin.
That means, a macro plugin module type for JIRA needs to be implemented by Atlassian for Atlassian Connect (and then develop a JIRA Syntax Highlighter making use of this Atlassian Connect macro plugin interface).
I have created a corresponding request. Please vote and comment for Macro plugin interface for JIRA Cloud!
Cheers, Holger