"AB Nappy/Diaper Harness and Reins/Leash Sets, prevents removal of Nappies/Diapers etc, £29.95, you can add a steel upgrade, wrist restraints, locking back buckle etc if desired"
The traditional child-style harness has two more straps which drop vertically (on a standing adult) from the centre of the chest, pass through the groin (one on each side of the diaper bulge) and then link up in the centre of the wearer's back. When properly tensioned, the effect is to prevent removal of the diaper, because the wearer cannot lower the plastic pants enough to gain access to the tapes or pins. Of course a fully mobile adult will get past this, but if the hands are less mobile, then the wearer cannot wriggle out of the diaper, or lower it using (for example) the edge of a table.
To restrain the hands, the harness can also be fitted with wrist restraints (at extra cost) which grip the wrists at chest height. This really prevents removal of the diaper! I find that wrist restraints are not necessary, as the high-chair straps which come with the harness can be used to make short leashes (wrap strap around wrist, push clasp through loop) and then clipped onto the existing D shackles under each arm. This allows some movement but not enough to reach diaper area.
The harness itself, and the wrist restraints (or leashes) can be padlocked. Again, padlockable clasps are an extra cost. Metal clasps are another optional extra, but should withstand the most energetic tugging.
So, with a lot of money spent (up to £100 or $200 for all of the options) you can have a harness system which is certain to prevent any attempt to remove the diaper until the keys become available. Be sure to leave sufficient movement to be be able to insert the key into the first padlock, and then turn it!