OTA functions are available in user code, but not documented very well. I posted a (now non-working) example of flashing OTA via HTTP. I haven’t had time to revisit this code, but it could probably be revived with some effort. In theory you could flash new code OTA via Particle cloud, and then use a custom routine to flash OTA outside of Particle Cloud in the future.
EDIT: Original post here. OTA over HTTP working example!