If they learn enough to get physical access to the machine that runs the script then you're already pretty much screwed, obviously. At that point the encryption is merely there to minimize damage.
Your machine could be confiscated coincidentally if you're involved in other things that make people angry. In this case, they may just gloss over any content irrelevant to the charges at hand. Anecdotes I've heard seem to corroborate that mode of operation. In such a situation, the email archives and/or the backup script may never come into play anyway.
You could also not use a script and do it by hand every day. It's just a trade-off of risks; you either do it manually each day and allow yourself to accidentally forget a day, or get lazy and let it go for weeks, or whatever, and keep that data in the hands of a third-party where you really have no idea what's going on with it, or you can put a script on your machine, where local analysis of your HDD could reveal you.
Also, there'd obviously have to be some unencrypted parts of this, but you could probably encrypt the part that contains the actual email address. You can just point a cronjob to a script that decrypts the real script and shreds it to your paranoia's satisfaction when done.
If the script can actually run fully automated (i.e., no passphrase needed), then that'd probably be a problem if someone got a hold of your workstation. So it really shouldn't be totally automated, but mostly automated would still be good.