I'm under no doubt that your question makes perfect sense to you, but as an outsider reading it for the first time...well, even several times over its still a little confusing.
I'll go through it bit by bit, that way you can see what needs further explanation, for me anyway.
...Any of you gurus know of a way to automate a process...
It has been known to happen, yes......so far so good!
...a process to identify the grey scale of an individual pixel...
Using what method?
There are several methods to convert colour to greyscale, all of which give a slightly different result. Some are built into PS, some are not and would need to be written into a script. Depending on what method you need to do this will determine how this "automation" will be executed, so its important to know.
...do a global replace for all such pixels with a specific image file?...
Now, you've turned this statement into a question which kind of indicates you are not sure yourself if it's the right thing to do. To me personally this reads that you want to replace all the pixels of a certain 'greyscale' value with a whole image file.
If we knew what the 'goal' of the process was we could ascertain for ourselves a way to do it.
What you describe cannot of course be done as I read it, so this is a bit confusing as to what you mean.
...That process would then be replicated with a different image for each range in the grey scale...
Range in the greyscale?
What 'range' would that be then?
How is this 'range' defined within the 256 available?
Also, what 'image' is being used here and for what purpose?
Do I assume there will 256 different images?
How does the image get used in the 'greyscale' scheme of things?
Where are all these images?
...A little weird, I know, but so am I...
I understand completely.
With such a 'specific' process you are going to have really nail down every little detail in order for anyone to fully understand what needs to happen.
Automation need not be difficult, but in order for anyone to even start to think how to 'Automate' something they must first have a rock solid knowledge of how it happens 'manually'.
Any chance of a manual example?
I'm sure we can come up with something for you....once we know what the process is trying to do.
Regards.
MrToM.