There may be some distinction here I'm missing. In addition to this domain aliasing feature I haven't tried out, Fastmail.com is hosting my domain. Right now it's got their standard settings:

       ancell-ent.com  Type=MX   TTL=3600  STANDARD_MX          (standard email - e.g. user@ancell-ent.com)
     *.ancell-ent.com  Type=MX   TTL=3600  STANDARD_MX          (subdomain addressed email - e.g. user@mail.ancell-ent.com)
       ancell-ent.com  Type=A    TTL=3600  STANDARD_WEB         (main website - http://ancell-ent.com/)
     *.ancell-ent.com  Type=A    TTL=3600  STANDARD_WEB         (subdomain websites - including http://www.ancell-ent.com/)
  mail.ancell-ent.com  Type=A    TTL=3600  STANDARD_MAIL        (webmail login - http://mail.ancell-ent.com/)
       ancell-ent.com  Type=CSV  TTL=3600  CSV_DENY_ALL         (no email sending machines identify as ancell-ent.com)
       ancell-ent.com  Type=JABBER TTL=3600  STANDARD_JABBER      (standard chat server - chat.messagingengine.com)
   wap.ancell-ent.com  Type=A    TTL=3600  STANDARD_WAP         (cellphone WAP service - http://wap.ancell-ent.com/)
       ancell-ent.com  Type=DKIM TTL=3600  STANDARD_DKIM        (dkim key mesmtp._domainkey.ancell-ent.com)
       ancell-ent.com  Type=AUTOCONFIG TTL=3600  STANDARD_AUTOCONFIG  (mail autoconfig)

But there's also a "Custom DNS - control all records published for the domain" feature I haven't tried out yet, since I don't at this time have anything else for my domain publicly accessible.

Ah, maybe you're referring to how this translates to Fastmail mailboxes. Specifically a "virtual domain" concept where aliases are established, and I have both the wildcard * and hga at my domain aliased to hga at fastmail dot fm?

If the above is what you're trying to do, how is it user unfriendly? If not, out of curiosity and maybe a need to do this myself in the future, what are you desiring to do?

It's user unfriendly because the @fastmail.com is your actual email address. It's user unfriendly because you have to know that. It's user unfriendly because if you enter settings incorrectly into your mail app, you'll be sending email as @fastmail.com.