To use Google Talk with a third-party email address--despite the abilities of Android and iGoogle--it seems you really must add Gmail to your account. If you visit gmail.com while logged in to your non-gmail Google account, you'll be presented with a form to sign up:
Welcome to Gmail
You're currently signed in to your email@example.com Google Account. If
you complete this form, you'll be adding Gmail to this account, and
your Gmail address will become your primary account username.
The "primary account username" part is what had been scaring me off, but it turns out:
- You can still sign in to various Google properties with your original
- You can still hand out your real email as your Google Talk address.
- Your buddy list will remain intact, and people who you chatted with prior to adding gmail will still see you in their list.
CAUTION: Choose your "throwaway" carefully; it will forever become a part of your Google account (even if you remove it "permanently"), and it is likely to be seen by your Google Talk contacts even if they add you via your original/real address. For this reason you should consider choosing something along the lines of
firstname.lastname@example.org as your new address
After completing the form and confirming the account, your Google Account settings will show your new Gmail as Primary, and list your original email address as secondary. Now you can configure your previously un-cooperative chat client with your new @gmail login (if it doesn't work at first, you might need to sign in to Gmail proper at least once).
At this point, you can actually remove Gmail from your account
To do so, visit https://accounts.google.com/b/0/EditServices and select "Remove Gmail permanently". Once you complete the process, your account will show your original email again as Primary, and list your gmail as an "Additional login". Mail sent to this address will now bounce, but you can continue to use it as a Google Talk login.
Whether or not you remove Gmail after adding it, you'll still show up in buddy lists with your new @gmail address. As cautioned above, "Remove Gmail permanently" isn't completely true!
While this may not be the perfect solution, it accomplishes the goal of being able to hand out your real email as your Google Talk address with grumpy client software even when your domain isn't on Apps. In the end I opted to keep the gmail address on my account; I actually appreciate the side effect of having a searchable archive of my chats (a feature that can be disabled in Gmail settings).