Cabotage only relates to a third country OPR impinging on the domestic market of another country, so shouldn't be a player, however I have heard it crop up against private flights, albeit G-reg carrying a Russian owner.
The Russians should be experienced enough now to recongise M- as solely private, but it helps to have a sensible Russian agent on your side in the event of a problem. If your pax has been manifested from first entering Russia to leaving this will also help, but Russians may be suspicious if the pax just pop up in St Petersburg, get flown to Moscow and then you depart empty.