If this does work without issue, then I think all deployments should have at least one relay with a port that is either 80 or 443 just so that clients with restrictive outbound firewalls could report in, particularly for an internet facing relay.
For this proposed relay using 443, I would give it an affiliation group that no client has so that it will not be apart of the * pool of relays, and then set it up as the failover relay. ( We use automatic relay selection everywhere ) With this configuration it should only ever be used by clients that fail all other relay selection, which would be a good indicator that those clients are having an issue with the other ports.