If you're using a condition, you can use the following to get a handle on the region ID in your PL/SQL code: wwv_flow.g_plug_id(wwv_flow.g_current_region_id)
This will unfortunately not work for PL/SQL executed as part of authorization schemes; only conditions.
Also, this will only work in APEX 4.0; it is not supported in APEX 3.2 & earlier. Thus, you'll have to wait until its released before you can use it.
Even with this approach, you'll have to keep in mind that Region ABC on Page 1 of App 123 will have a different ID once you export & import to another application; thus, you should try to store any rules that you will use in tables, so that things don't stop working when you promote your application from one instance to another.