I was afraid that would be the answer ;-).
As far as I understand it, I can’t let the normal agent connect to a.example.com and the proxmox agent to localhost. Both use the same domain name, right?
So I would need two static tunnels and both connecting at localhost, so it would be something like:
localhost:2222 => a.example.com:22 (ssh) (normal agent)
localhost:8006 => a-example-com:8006 (proxmox agent)
I’ve only got one Proxmox Server standing around, so I can handle that. But that seems overly complicated for companies which monitor a lot.