As well as 4 years technical experience with .Net , Ruby on Rails,and Java, I also have experience building a custom ORM for one of the long term clients.
For a previous client I built a database mapping and code generation tool for their new booking system product, which was based on Rails C#, MYSQL,MSACCESS and XML which follows the principles of an ORM. This was used as the foundations for the whole project.
I am NOT going to recommend that you use something similar. We built the custom ORM for reasons that made sense at the time and I have come to believe that it may have been better to use an out of the box solution.
However, the project has given me an excellent understanding of the principles involved in building and managing an ORM and the technical advantages and disadvantages of choosing one. I believe I would be in a good position to analyse your requirements and advice on the best manner of proceeding.