With this question you really hit the jackpot...
I assume that the content that you are trying to play is not in the native apple format, but more like ordinary .avi and .mkv files?
It is possible to install the air video server software (v. 2.0.00) on the readynas ultra (also intall the ffmpeg sw 1.3 aka 2.0.00). Now you can conntect to the readynas ultra directly with only one problem => the movie stops every 5 seconds to load and convert. According to another forum member, this is caused by the processing power which is too low on the readynas ultra (he claimed that you needed the readynas pro at least)
==> This is a no go solution, unless somebody comes up with a great idea
, By the way, the Plex solution has the same issues as the air video solution, processing power too low.
A second solution which works fine for "lower resolution" movies (i'm speaking of a maximum of 1.5 GB avi files) is using the Oplayer HD (free version works great, but you have a small banner at the top of the page). You don't need to install anything on the NAS. Plays about everything (also large mkv's of 10 GB, but than it starts to stop frequently to capture and translate the strean). Make sure to increase the buffer size to its maximum (setting can be changed in the parameters and I believe that 20MB is the maximum buffert size) => If only this buffer size could be increased, this would be the ultimate solution!
Any other idea that you come up with to play movies directly from the readynas ultra on the ipad are highly appreciated.