Assuming your plugin won't be doing the importing, then
* would it be possible to require metadata or something to make it easier for your plugin to detect imported photos?
For example, could you ask users to assign a keyword "Imported" upon import, then you only have to look at photos with that keyword, and when you find them, clear the keyword and process..
Obviously, you can (continuously) go through every photo in the catalog, and see if it was there before, but it's kinda cpu intensive..
Be aware that Lr does NOT stop background tasks when you reload the plugin, so they can gang up. - so make sure you have a shutdown module which sets a global shutdown variable, and check that variable in your background tasks - if set, return from the task.
Likewise, make sure background tasks are stopped if there is an error (use function context cleanup handler).
Without such precautions, background tasks can really be problematic..
PS - A great way to have a background task (if LR 5+ can be a requirement) is to use a floating dialog, which represents your background task - when dialog box disappears, so does your background task..
Rob