This news is weeks old. I got bored of this ‘Mario Mods’ project within a month, but I’ll try to rejuvenise now, so here’s some old but awesome news :)
The guys at youtube channel NSMBHacks have tried out some of the beta stuff still left on the SMG disc. By replacing existing levels (that you can access in the game) with the beta levels, they’ve made some pretty cool videos. Check them out below. Note that these planets were never finished, so might lack proper physics, barriers, solid areas etc:
Beta Planet
Star Man Fort (Beta Level)
Star Man Fort 2
GlassyTwinsPlanet & Pole Unizo Zone
More unused planets & objects
And here’s one by jimbo3390: Beta PlanetDisk