I am not exactly sure what you mean.
To be clear I never said anything like "load a reaction to an impulse".
that makes no sense to me.
Like you said, an impulse responses in this context is basically the very last component in your signal chain, the "Guitar Cab's characteristic captured with a microphone".
In practice, these IR's are basically very short .WAV files that are just like any files on your computer.
In any modern DAW you can quickly go through lots of IRs (.WAV file) in your guitar signal chain simply by loading and swapping them in any IR loaders.
All for finding the sound that you desire.
POD HD works the same way,
it has a signal chain, from pedals, preamp, poweramp to cab+mics, and from my understanding,
all the CAB-MIC that you see in your POD HD are just .WAV files, digitally located inside the flash memory.
As for third-party IRs, some IR libraries have very large pool of tinny variations of the same cab+mic.
For example you can find IRs that are Mic-ed on axis, 45 degree on axis, off axis, 1 inch off axis, 1 inch away from the cab and so on... all with the same cab+mic combination. (http://www.redwirez.com/)
On the current POD HDs, you only have a very few options.
someone said the POD HD's are not capable of loading your choice of IRs but I just don't know enough about why that is the case.