I plan to do the same thing with my forum, but I'm still not there yet.
However, I did integrate Blank Template Mod with ezPortal on phpbb2. It was relatively easy and I assume it'll be easy to integrate it with Board3 portal on phpbb3 as well. All I had to do was copy the code specific to the Portal on portal.php over to template.php, and then copy the portal_body.tpl code from the template to template.tpl. EXCLUDE any code for the news bits or anything in the center column that you don't want to keep. In some cases, it was finicky based on what order I combined the php code together in, so there was trial and error involved.
Blank Template is terribly simple, all it's doing is calling a title and a body (ok, and some perms). Adding the portal code to the template.php file was simple; MIND YOU I don't know if it's possible with Board3 because I haven't looked that closely at the main portal php file. I've actually spun off quite a few custom portal templates; I have 3 of them counting the one on the portal itself.
Here's what it looks like:
Portal:
http://www.airsoftpacific.com/
Blank Template Mod:
http://www.airsoftpacific.com/content.php?page=38
EasyCMS and Template Mod:
http://www.airsoftpacific.com/cms_view_ ... php?aid=22
Hope that helps for now; once I get down into the nitty-gritty of trying this with Board3 on my phpbb3 implementation I'll be sure to share my findings.