Uhm, well, I think the callback is available for all plugins and components already (and implemented in a bunch of them, with fallback to default_page_type_list).
The key is that right now, add_block_at_end_of_default_region() is using simple pagetype as default pagepattern and that leads a lot of times to wrong (non-supporter) patterns in DB.
So the proposal, that was applied by 67a60db9e76c132a2af0 @
MDL-27829 exclusively to mod-xxx-* pages, is about to guarantee that the pagepattern to be inserted when one block is created is always a supported one (by looking to the available patterns).
And do that to all the blocks created, at any page. So generate_page_type_patterns() will be called always and, if the simple pagetype is not a supported one, the system will get the first available pagepattern as default one.
for example, right now, creation of blocks is leading to the insertion of "wrong" pageppaterns in DB (course-report) and that leads one (translated) warning when editing the block about the pattern being changed because the existing one is not available anymore. With the proposal working, the insert will guarantee that the created record already has one valid (supported) pagepattern. So the warning won't be shown.
That's all. Note this is tidily related with 28149 and depending of that, this will need some change and vice-versa.