Came this close to sticking with the Thunderbird 3 Alpha/Shredder series, but search tipped me back to Mail.app and emaildiscussions.com solved my problem.
Quoted for posterity:
“For the sake of continuity, I’ll give the PlistEditPro steps here.
a) Close Mail.app. Make a backup of your com.apple.mail.plist file by copying it to the desktop or something.
b) Open com.apple.mail.plist in PlistEditPro
(If you’re not familiar with the property lists, do not modify anything else, except for those here below).
c) Scroll down to mail accounts, and navigate open.
d) You may have 0, 1, 2, etc.. depending on how many mail accounts you have set up. 0 is the local account. Navigate open the main mail account that you primarily use. (scroll down to user name and host name to identify which account it is).
e) Single click on email addresses and then click on NEW SIBLING.
f) Give the name: EmailAliases, and choose class Array
g) Then single click on EmailAliases, and then click on NEW CHILD.
h) Give the name 0, and choose class as Dictionary
i) Single click on 0, and then click on NEW CHILD.
j) Give the name: alias, leave class as String, and give value as the email of the identity you would like, eg: email@example.com
k) Again single click on 0, and then click on NEW CHILD.
l) Give the name: name, leave class as String, and give value as the name you would like to be when sending out, eg: John Doe
m) Now you can single click on 0, copy and paste there itself (apple-c and apple-v), and you get a second alias automatically set up.
n) Modify it to what you’d like, etc..
o) Dont forget to save once you’re done, or you wont see any changes. Say yes to replace.
p) Open mail.app. Everything will be as before. Click on new message, You get the option to choose from the identities you just created.”