Have you tried JustBrowsing? Seems to be what you are looking for.
[X] Installable on a USB device
"Boot from CD-ROM, USB, hard drive, I-ODD, or Virtual Machine."
There is an USB boot option on the Start Guide that guides you through.
[X] An up-todate browser (preferably Chrome, Chromium or Firefox)
"Releases are built with the latest version of Firefox (stable) and Chrome (dev)."
[?] Flash installed
I don't know if comes installed, but they cite Flash on Extensions.
[ ] Bonus: Java installed
I'm not aware of it; didn't see anywhere on the website saying anything about Java.
[X] Small, fast, light-weight
While the ISO image is not that small (~700 MB), they claim that the system is pretty lightweight:
"Designed with old machines in mind. Don't throw out your old computer, put JustBrowsing on it!"
[X] Architecture: x86
Supports both x86 and x64 architectures:
"Based on ArchLinux i686 (32-bit). Now available optimized for x86_64 (64-bit) too."
[X] License type: Any
IDK about which license they use exactly, but they claim:
"The JustBrowsing project is free and open source."
Maybe they use different licenses for it part of the whole solution.
"I'm looking for a maintained version of BrowserLinux 501. Sadly, it is no longer up-to-date."
JustBrowsing seems to be rather active. They have on the What's New page a section for Roadmap, Recent Changes, Project Goals and Changelog.
If you check the changelog page right now you can see that the latest version (nightly build) is from just a month ago (2014-05-25).
Not to mention that they are also on several social networks: Reddit, Tumblr, GitHub, Twitter, Google+, Facebook, Pinterest and StumbleUpon.