Can't speak to that because I don't understand this use-case, but AFAIK it should be possible to modify an org agenda as long as emacs is running somewhere. It

Can't speak to that because I don't understand this use-case, but AFAIK it should be possible to modify an org agenda as long as emacs is running somewhere. It could write the current agenda to a file (in a s3 bucket, for example) with the function org-agenda-write, and the application on a phone could read the agenda and "push" the changes back, and an elisp function could handle modifying the actual agenda. Wouldn't be surprised if this was already posted on the mailing lists as a POC or something.