Isn’t this pretty much impossible as things stand? There is no public API for reading iMessage information and all third-party apps are sandboxed on iOS.
Are you counting on some kind of policy change from Apple?
The minute that changes / the minute further sandboxes come in to prevent reading of the unencrypted chat at rest, these are at risk.
It does seem like even automattic is counting on putting more people into a common address-space before that happens so they can continue to build out their network and support protocols that are platform and device independent.