Well I did write into slack before my failure of not access the file data in memory. With a kernel module (2.6) it worked like this:

1. Save the origial size of the file
2. Open the file to write and append
3. Write something
4. Close file
5. Open file for read
6. Change the size file back to original
7. Close file
8. Invoke the write_inode_now() function

I am not sure why this approached was overlooked as an option, but it did work very nicely. And I had to use the the get_fs() and set_fs() functions:

oldfs = get_fs();
set_fs(KERNEL_DS);
...
set_fs(oldfs);