Using Next Header with IPv6

This is more to implementors or people that understand network stacks...

We provided in 7401 a Next Header field as all good IPv6 payloads are 
suppose to do.  But we really have not used it in front of 'real' IPv6 
payloads like ESP, TCP, UDP, or IPnIP.  Or have we and I have just 
missed it?

Anyway this is for faster mobility; looking at 5206-bis and IF the 
payload is small enough to fit in a HIP UPDATE, to send the Mobility 
UPDATE along with the actual first data from the moving host.

This question is primarily 'will the stack handle this'?  That is, HIP 
gets to payload first.  If validates the UPDATE, changes the necessary 
bindings the either:

     pushes a revised frame without the HIP UPDATE back into the IP 
interface
     or sends the Next Header off to where it belongs.

I am not a person that has dealt with the insides of the OS(s) to know 
how this would be done or if it is really practical.

Oh, and what is the length of the basic HIP UPDATE in 5206-bis?  I can 
do my best calculation in a bit, but if someone has it...

thanks

Bob