How to pass parameters between these two functions? Do you use global variables?
There isn't any direct way. If you need to remember something from one to the other, you would need to store it in a global map with the filename as key, or something similar.
Keep in mind that the plugin may be called from multiple threads and for multiple files in parallel, and that DVP_LoadText will not always be called after DVP_IdentifyFile. DVP_IdentifyFile may also be called multiple times before DVP_LoadText, depending on why Opus is doing the query.
So, if you do store information between the API calls, you'd need to ensure it wasn't a large amount of data or was cleaned up on a timer in case the second call didn't arrive.