What the hell is it?
Now your other question is, what the hell is BTRON and TRON? TRON (The Real-Time Operating System Nucleus) is a Japanese effort to define a universal standard from top to bottom; scaling from microcontrollers to mainframes, and CPU design to operating systems. While these ambitions were ultimately scaled back, TRON remains relevant, primarily as an OS in deep embedded, such as engine EDUs. As such, TRON as an OS isn't really an OS, but rather a set of specifications similar to POSIX. The BTRON standard then defines a GUI OS based on TRON.
The interesting thing about ChoKanji is the highly document-oriented GUI. Templates, embedding, and dragging make up the core UI metaphors. For example, OS patches are distributed as README files, that happen to contain the OS patch, sub-documents, and spreadsheets representing tables inline. You can double-click documents or tables and open them up in the application, or drag them like any other object. To install the OS patch, you just drag the inline object representing the patch to the system settings aplet.
You could also drag templates into cabinets or documents and embed them inline. The browser documents are just objects that represent URLs, and open up the browser to that URL when activated; so hyperlinks and bookmarks in the OS can be implemented with inline browser objects. Effectively, it makes the lines between what's a document and folder blurred from a user standpoint.
Installation and patching
In my experience, it works best in VMware, though it seems also fine on Virtual PC. Unfortunately, it won't seem to work on Hyper-V. On VMware, you even get clipboard sharing!
To start with, you need to acquire a copy of ChoKanji. They still sell it!
Then you need to install the OS. Pop in the CD, and then begin partitioning. In the bottom cluster in the bottom, the top middle is create partition. Accept defaults, then click the bottom right button, which is install. Accept defaults, which will format the disk, set up a bootloader, and install a full software set. Once done, click the top right button to reboot.
Once installed, usually video and Ethernet just work. A setup window will appear on first boot, and will ask you how you want the keyboard to work for shortcuts and kana input. You can enter what you please here. (Remember that it defaults to a Japanese 104-key layout, so symbols may not be where you expect, and by default, kana input is on. Press caps lock to switch to roman input.)
The easiest way is started after that is to install the latest patch and then an English support patch. Install the 4.540 patch first, then English. Go to the browser and enter these URLs. (If you can't find where the _ is on Japanese keyboards, you can try to go to the support page, and find the pages there.)
Once you're at these pages, before you download, switch the file type to archive. (That's the middle button on the dialog, and then third option in the drop down.) Once it's done downloading, open the archives, then extract the README by dragging it out onto the desktop folder.
For the 4.540 patch, the patch object is further down in the document. Open the system settings applet by opening the menu, then the Utilities submenu (小物; bottom option; ChoKanji doesn't mark submenus - alternatively, you could open the Utilities cabinet on the desktop) and then pick the second option. (システム環境設定) The last tab will list system file versions; drag the patch object onto this, and it will prompt you to install.
For the English patch, repeat the process; albeit drilling down into the English and then Installation documents that contain the patch object. Once installed, you'll need to go back to the utilities menu and pick the language switcher. (言語切替; bottom option) Pick the second option, then click the switch button. It'll do a quick reboot, and then the system will be in English. Once installed, you can check the after-patch English language documents for an English language template library.
What software exists?
I'm unsure of much else of TRON and ChoKanji. I'm aware of a Mozilla port, and GCC definitely exists.
Ultimately, the OS has a lot of interesting ideas going on - the closest Western things I can call it close to is RISC OS, (the UI is sparse, also emphasis dragging, and the anti-aliasing feels like it) OS/2 Workplace Shell, (the emphasis on templates) or Windows 95. (OLE2 embedding is similar UI-wise)