The IDMLlib is able to extract any information inside the IDML. You are not limited in any way to transform this information into something else. For example you could use java2d to render a spread from the IDML or you could use a PDF library to create a PDF from the IDML document. To answer your question: Yes, the IDMLlib provides the foundation to develop a viewer for IDML.
Best,
Andreas
p.s.
The IDML format is available since CS4
edit:
iText would be one PDF library another one is www.pdflib.com which is an extremely fast library to create any kind of PDF but is also a commercial one. From a "what is possible" point of view, the pdflib is able to render a lot of stuff without much hassle but what will be very hard to do is stuff like soft shadows, image blendings. You will also need additional software to convert EPS to something (PDF maybe) that the pdflib can work with, because EPS files are not supported by the pdflib. Fonts could also be a showstopper, if your idml document uses Mac fonts (Font suitcases, .dfont). All other fonts are supported very well (Truetype Fonts, OpenType Fonts, PostscriptFonts). I don´t think it will be possible to create a viewer that can render every document 100% accurate but it will be possible to create a viewer that can render a lot of documents very accurate. But as always mileage may vary, best would be to get a eval from both libs and give it a try.