To my knowledge, accessing individual layers is possible only from automation plugin but it cannot access individual pixels.
But automation plugin can call other plugins. Take a look at automationfilter example in SDK.
It calls filter plugin “hidden” also from SDK which can access pixels.
Hope this helps,
The ExportRecord of the plugin type contains a pointer to the documentInfo.
The layerDescriptor can then be iterated over for every layer in the document.
You can use the ChannelPortsProcs to read the data from those layers.