Hi Mark,
I took a bit of a look around and the only solutions I could find to problems that looked similar, involve js, like this one:
(I know it’s not the same problem but seems like a similar solution could work for your problem).
I would try maybe reposting under "custom code or maybe the person that helped in the above post - @samliew - might be around and up for helping solve a problem 
Sorry I couldn’t provide more help.
-Jeff