Devel::Cover only seems able to report on the test suite for a single platform, and not an aggregate coverage across multiple platforms (correct me if I'm wrong)

Paul Johnson describes how to do it here. This has come up a number of times on the perl-qa@perl.org mailing list. On Paul's to do list is to set up a database to collect coverage data from decentralised machines (running different versions of Perl on different platforms) along with a web interface to view this data. I think he is looking for volunteers, so you might like to contact him. ;-)

Oh, and though I'm sure you've thought of this already, one can employ mock objects (for example, Test::MockObject) to improve coverage on all platforms (and make the overall design more "testable").