Which version of Moodle are your using.?
In 2.1, I was poking around to see what was going on with my permissions and capabilities in different custom roles. It had a couple of reports in site administration >Users >Permissions.
I first looked at the Capability report. This is tells what the permissions are for each role defined in the system.
Next I looked at the Check system permissions report. I could create 1 user and assign that user a system wide role. If I was in the planning phase, I would do this on a non production site, so I could try out my custom roles.
Never overlook the good old fashion copy and special paste (Unicode Text) into Excel technique. I just did it for a user in 2.1. Created a 2 column 464 row report. Bottom of the worksheet looked like:
|Edit a block's settingsmoodle/block:edit
|Manage blocks on a pagemoodle/site:manageblocks
I am sure you can find some clever Excel user to come up with some formula or do a cut and paste of the second column to create your report showing every role's permissions at the system level
Someone else can chime in about earlier versions of Moodle. Or perhaps some clever SQL query. Hope this helps.