Reading the section on backups, resleeving an Ego Bridges, I am a little unsure whether the default system requires synthmorphs to utilise an Ego Bridge in order to back up the cyberbrain's state.
The relevant passage is:
The ego bridge then builds a digital copy of the person’s brain, which is stored away in the service’s highly secure, off-the-mesh, hardwired data vaults. In the case of pods, the ego bridge scans the biological brain bits and also accesses the cyberbrain to copy the parts of the ego residing there. For synthmorphs, who have no biological brain, the process is much simpler, as it simply requires accessing and making a copy of their cyberbrain.
While the part about cyberbrains is in a paragraph beginning with mention of an ego bridge, it doesn't seem clear to me that one is or should be required in order to back up a cyberbrain.
This is further implied in the Resleeving section, which both allows for instantaneous evacuation of a cyberbrain and for instant uploading of a backup into a cyberbrain, apparently without the need for an Ego Bridge.
Ruling this myself I am inclined to allow backups and resleeving to be made on the fly from synthmorphs without the requirement for an ego bridge, but it would be good to get an official clarification.