You should note that it's a new feature that's being pushed on the Ultra Series but as the NVX and Pro are x86 ReadyNAS and would work with pretty much the same (if not exactly the same) code they've made it available for all x86 ReadyNAS.
The Sparc platform (NV+ is on this) is quite different. There are a number of possible reasons why it's not coming, here's just a few:
1. New feature being pushed for the x86 platform, primarily on the Ultra Series. Can use this as a feature to differentiate the x86 line from the older Sparc line. Can't expect all new features to come to all the older products.
2. Source code may not be available that can be compiled for Sparc or be too difficult to backport.
3. Additional code has to be written and compiled for Sparc separately to x86. This can take a lot of additional time due to the differences between the Sparc and x86 RAIDiator OSes.
4. Slower CPU in the Sparc ReadyNAS may be an issue (whether this is an issue or not depends on how TiVo archiving works)