<p>Is there an online resource somewhere that maintains statistics on the install-base of Java including JRE version information? If not, is there any recent report that has some numbers?</p>

<p>I'm particularly interested in Windows users, but all other OS's are welcome too.</p>

The source code seemed to be geared for an executable, you might need to rewire stuffs a bit so it would build as a DLL instead. I don't have much experience with Visual C++ but I think it shouldn't be too hard with some research. My guess is that someone might have had made a library version already, you should try Google.
Once you have tesseract-ocr code in a DLL file, you can then import the file into your C# project via Visual Studio and have it create wrapper classes and do all the marshaling stuffs for you. If you can't import then
DllImport
will let you call the functions in the DLL from C# code.
Then you can take a look at the original executable to find clues on what functions to call to properly OCR a tiff image.