I've toured a fair bit (always with Brooks saddles) and have never felt a need to secure the seatpost.
Contextual awareness has always been my main security technique, and adjusting the level and type of precautions accordingly. I don't see any advantage to stashing stuff, especially anything valuable within the bike.
If the situation requires it, have most of your valuables on you, or with you. Keep some small cash reserve or second credit card elsewhere buried in a pannier. Splitting the stash increases the risk of losing some of it, but decreases the risk of losing all of it.
Record important numbers/docs on your phone in a hard to find file or as photos. Have most of your 'wealth' stored online, or as debit ie in a form least likely to be physically lost or stolen.
One trick I've never tried but sounds neat, is to fill Allen key bolt heads with wax. Stops opportunists apparently. Heat with a lighter/match/hot water when you want to clean them out.
Regards Graham
Canberra Australia