AFAIK, almost no one has managed to do real HA with Asterisk without modifying core Asterisk.
Matt Jordan has demoed at of the VoIP conferences where with a modified version of Asterisk he was able to do true HA, but the code was never made public.
The only company I know that was able to pull this off without modifying Asterisk is Telium. However, we have reached out to them several times over the recent years and we were never successful getting a hold of them. So I am not actually sure how they do it.
I really wish that Sangoma decides to revive the SCF project.
I mean, from what I understand - correct me if I am wrong: as of today, the core Asterisk plumbing can be aware of call details happening on other systems, like state distribution etc. However, I don’t thing there is anything is Asterisk that would allow you to (re-)create an INVITE with the local/remote URI, CSeq, tag, call-id etc.
Newer generation stuff has more state separation to allow it to happen, and the foundational aspects in PJSIP exist. In a purely PJSIP (non WebRTC, non TLS)+ARI environment it would be easier. If you involve Asterisk dialplan applications into the mix then you’re at years of work probably - or more.