Hello Will,
I just tested your suggestion and created a button that trigers an eventhandler OpenLink with defaul value maps://.
On clic the browser is lanched with https://myxxxxxx.crm.ondemand.com/maps://
It seems that C4C is interpreting the uri as a relative path.
I can't find any parameter to force it as the full path.
When I enter somethin like https://google.com as default value, this is not concatenate with https://myxxxxxx.crm.ondemand.com
Best regards,
Benjamin