The ability to add domains to a development environments on Platform.sh Grid is currently not supported, sorry.
There has been discussion of this as a feature request - “Vanity domains”, but it’s not currently possible for self-service or grid clients. For Dedicated accounts, this can be requested through your onboarding or account manager through a ticket, it may require some work if you manage your own SSL certificates.
Even if you managed your DNS to point
dev.myproject.org at the platform.sh outer routers, it would not be directed internally to reach your project. Only registered production domains are routed in.
You can add any number of aliases to your production instance at
Settings -> Domains however.
Some people have reportedly had success with setting up their own pass-through proxy to support this sort of effect, in cases where it was wanted for firewall or VPN rules, but that is not supported by Platform.sh support - you are on your own if you wish to set that up.
Alternatively, you may be able to set up a simple redirect on a placeholder domain from your cute URL to the ugly one, if you just want to be able to share a short or memorable link.