Well you said that Arch would fail LSB because it didn't have a converter, which seems strange if - as my link appears to show - it is capable of using RPM natively. That is why do they need to convert .rpm packages if they can run RPM and install them direct? Not that I'm saying there wouldn't be any work to do but the fact that people are already using .rpm via RPM (the binary program) would appear to show it's not a huge obstacle.
http://refspecs.linuxbase.org/LSB_3.1.0/LSB-Core-generic/LSB...
To me says that the distro can use any package format it likes ("The implementation itself may use a different packaging format for its own packages") as long as it can handle .rpm packages ("Implementations shall provide a mechanism for installing applications in this [RPM] packaging format"). So if Arch shipped RPM it would appear [superficially] to meet the requirements.