Just in case anyone else stumbles across this. We gave up on shopify - they limit openid connect to google only, unless you sign up for $2000++ p/m with shopify plus. There are some 3rd party plugins for shopify, and that led us to a vendor who also had a plugin for Magento, so we decided to ditch shopify for magento. Fortunately our project is running so late that this was still feasible as not much migration work had started that was shopify specific.
It seems odd to me that openid connect is not just a standard feature of these platforms. Well, I guess in the case of Shopify it is - but they deliberately hobble it to google servers only in all but the top teir.
Adding openid connect in a plugin seems overcomplicated and not a good use of the current plugin api's.
None of this is Gluu's fault of course, and I'm only putting this here so other people looking to use gluu with an online store have a bit of informaiton about the state of it on this date (I guess within 12 months it may be different again).