Another potentially useful spin on filesystems is to make one that's explicitly able to manage text files. Whether you do it with a backing store (on-disk) implementation, or as a RAM disk, depends on your requirements. The characteristic of text files is that they are sequences of line-feed-separated entities, with the data on each line somewhat independent of the data on the previous or following lines. Consider a form-letter, where the only things that might change are the salutation, the account number, and perhaps an amount that's already overdue.
By creating a line-based filesystem, you could change the dynamic components of the form-letter without having to readjust all of the other bytes in the file. If you combine this with something like HTML or PostScript, you can start to see the potential.
The basis of line-based filesystems lies in the same technology used in text editors. An array of pointers-to-characters forms the basis of the “lines.” What the pointer points to is a complete line of text, and that's where you'd make your dynamic content plug in. The rest of the lines wouldn't need to change. By implementing it as an array of IOVs, you can pass this directly to the MsgReplyv() function that you'd use to return the data to the client in your io_read() handler.