To get it working on Virtualbox all you have to do is, before installation (or whatever part of it declines at the bios date), set the clock on your host system to match the appropriate date, and since Virtualbox has no bios of its own, this will trick it into thinking the date on the host is the bios date of the guest. To find out what date you need to set it to, you can find a comprehensive list of most of these builds, along with their details, screenshots, product keys and required bios dates and timebomb warnings, all on The Collection Book website.
4074 seems pretty cool and stable, I honestly think I might use it or a better one (if one exists) as a primary system after a while. It sucks that these themes don't exist outside of these builds. Slate and Jade look great.
Comments
To find out what date you need to set it to, you can find a comprehensive list of most of these builds, along with their details, screenshots, product keys and required bios dates and timebomb warnings, all on The Collection Book website.
Edit: Nevermind.
TCP8W-T8PQJ-WWRRH-QH76C-99FBW
More leaked builds https://archive.org/details/6.0.4039.0.lab06n.0308241954x86freclientprofessionalretailenuslb1pfreendeveloper/itl.cat_windows-longhorn-wallpaper_1267965.png
Some More https://archive.org/details/thelonghornarchive