Hi!
Has anyone integrated Defold as part of existing app? eg Android Fragment (not activity, Fragment 
) and UIView subview? Been digging the Internet and found nothing so far…
Also, is there a possibility to load assets from the file system using given absolute path?
No, this has not been done as far as I know and I am pretty sure it would require significant work. I would do the opposite and try to integrate additional activities in Defold instead.
What kind of assets? There is documentation about loading files here: Working with files