The front-end of the connector should work with Joomla 1.0.13. As you probably already know 2 major changes in Joomla 1.0.13 will cause some issues.
1) Password encryption changes. Th only issue this will affect is using the control panel to export users from Joomla to phpbb... since the encryption is different the phpbb password won't be correct. (v2 will have some code to help correct this)
2) Admin sessions have changed again... not sure what the big issue was but for some reason the control panel for the connector is more likely to indicate that an admin session is not valid. In the next alpha release of v2 I have changed how session checking works in the control panel to hopefully get away from that problem. Worst case you can comment out the line for initSessionAdmin to use the control panel. HOWEVER with that line commented out ANYONE can open the control panel from the URL... So if you comment it out to use... make sure to uncomment it when done.
Probably best to stay with Joomla 1.0.12 for v1.0.3. I have no plans to upgrade 1.0.3 to handle Joomla 1.0.13 better. Instead I hope to get v2 out of the Alpha stage in the not too distant future. Hopefully 1 more Alpha release then I can go to public beta stages.
Yes, I know there has been quite hassle. I waited long with upgrading joomla to avoid things like this. I went straight from version 1.08 to 1.0.13. Fortunately we dont get many users a day. So things can be handled manually. I understand that no patch is available soon. Pity, but we will manage.
I have the Alpha testers group closed at the moment but I think there should only be 1 more Alpha release prior to a public beta or final... So hopefully not too much longer for v2 to be out public...