OK. No need to touch the update_interval.

The script would do something like this:

1. decide whether to enable or disable automatic updates
2. set AUTO_UPDATE accordingly ("true" or "false")
3. execute
$ gconftool -s /apps/modest/auto_update -t bool $AUTO_UPDATE

(optional)
4. if/as wished, set a custom update_interval according to the AP/SSID
$ gconftool -s /apps/modest/update_interval -t int $MINUTES

Note that this will not affect Nokia Messaging in any way, in case you're using it (I use standard IMAP for some accounts and Nokia Messaging for other accounts).