Basic overview question:
I'm interested in creating a format plug-in that will open multiple image files and combine them, then provide one image to Photoshop for further work.
1. Can an .8bi plug-in receive notification of multiple files to be opened? (Not unlike Camera Raw). Or does one absolutely need to use an .8ba Import plug-in to do this? I would prefer .8bi.
2. Assuming yes, could the latter parts of the plug-in interface be used to open just one combined image?
1) An import plugin (8bi) doesn't get file information - it has to provide it's own open dialog. Only a file format plugin gets a file, but only one at a time. (ACR has some special magic)
You might want to take a look at the PhotoMerge and MergeToHDR setups with a script driving a plugin.