Thanks for an interesting post. I've just tried configuring a specific subdomain using your Pro-tip and I get an error message:

Only paid plans may bind custom subdomains.
Failed to bind the custom subdomain '2eb4ac40' for the account 'David Clarke'.
This account is on the 'Free' plan.

Thought you might like to know. Obviously the paid plan provides that capability - I was hoping to avoid the cost for the one-off project I'm currently working on.