A few questions on customisation with Piranha CMS.
Is there a way to create additional custom site-wide items similar to those from the site helper? http://piranhacms.org/docs/api-reference/site-helper
e.g. to be able to set a phone number used throughout the site but still editable in the settings section of the manager.
Is is possible to create additional custom items for a page, e.g. page subtitle, and for those to appear in the Information section of page editing? Creating a region for say a page subtitle seems overkill.
Is it possible to create a custom page type with a region that is a collection of HtmlRegions or similar? e.g. for the purpose of managing a set of FAQs or similar enumerated content?
Is is possible to define custom settings properties for a page type?
Yes, you can add regions to your site in the same way as with page types under Settings > Sites. You then add content for them under Content > Pages > Edit Site.
You can add simple text properties on the page type. They will be shown as single line text inputs under Properties when editing the page.
No native support, however the region body can be anything that can be serialized to JSON so you have to handle this in the edit view for your region.
There is no support for injecting fields into the page settings, however if it's important you can override the whole edit view for pages and do anything you like by placing a copy of the view in the manager area in your local project. You can find the views in the github repo in the 2.2.5 branch.
Regards
Håkan