777 or 340? As SloppyJoe said, there are stuff all trips they can use for SO training on the 340 without having to load an additional qualified FO or SO. Given there's a dire shortage of both, I'd suggest that's not going to happen and all 340 SO training will therefore be done on FCO or JNB trips.
Check & Training flights are always crewed first when the roster is constructed. If you're a line driver and they need the flights for training, then you'll miss out, regardless of seniority.