Due to the missing functionality of having "Priority per project" in JIRA (
JRA-3821), the JIRA default priority is useless when you have more than the default priorites. Which is a need if you have a big JIRA instance and many projects.
I think the main solution must be made in JIRA core rather than in Service Desk but respect the workaround from the linked issue will be a quick fix for the customer portal.
And maybe the SD Team can give the linked issue some higher rank in the Backlog.