Thank you all for your patience and feedback! This functionality will be included in SmarterMail 16.x. If you're participating in the BETA release, you can check this out in the current build that's available. Otherwise, stay tuned for the final release of SmarterMail 16, which is coming soon!
The new setting, Allow users to send from this alias, can be found when creating an email alias as a Domain Admin. When enabled, users can select the alias address when composing a new message by using the Send From dropdown. In addition, when replying to a message that was sent to an alias that you're part of, your Send From address on the reply will use the alias address by default. You can, however, manually change back to your standard address if you'd like to.
Also, any email alias that you're part of and that allows users to send from, you'll see in your account's signature mappings as well, so you can be sure that recipients know who sent the message.
Let me know your thoughts on this implementation!