1. Turn of data while roaming as you do, until you are in WiFi coverage. Then turn the GPRS radio off and data on. This will force data to flow over the WiFi connection to your Rim Operations Centre avoiding on-air and roaming charges. You will have to wait a short while until the BIS or BES realizes you are available and pushes your mail to you.
2. Have a look at LogicMail for Blackberry
. This is an IMAP client for Blackberry that can read your mail out of your server IMAP folders, and send mail out via SMTP. It will use WiFi if you want it to. I don't know if it will work with data service turned of, but if it is using WiFi it may.