Hi,
I spent some time reading up on Guided Tours and the recent changes that have been made then attempted to create a guided tour on a record producer in my own personal dev instance (though I am running NY so there might be some minor differences) so that I could have a frame of reference for the issue you're faced with.
It appears that for any guided tour running on the platform/standard ui, you're only able to begin your tour from a list or a form. When you set your tour type to run on the portal, this is when you have access to items in the service catalog, as you're able to select the portal and page on which you're running the tour. I understand that you're not looking to use the portal for this scenario, and it does seem to be a shame that there isn't a better way to access those catalog items from the platform when creating a guided tour.
I'm not sure how your instance is set up, but I did notice that you could set the guided tour to start from your homepage -- if your homepage happens to be the service catalog, or have a link to the sc, perhaps that could be a workaround for now? Is there any flexibility to have that record producer available through the portal? What kind of users are gong to be accessing the record producer?
Sorry this wasn't more helpful. I'm interested to know if you do develop a workaround for this or notice a solution I overlooked.
