SOD items are deliverable to parcel lockers (namely because SOD is related to the delivery address - anyone at an address can sign for it without having to produce ID, therefore whoever places it into the parcel locker can technically be the signatory - person to person can only be delivered to the addressee, so that's one service where ID needs to be produced and can't go to a locker).
The item being sent as a large letter aside, if memory serves, however, tracking and updates is 50/50 when it comes to parcel lockers, and dependent on who does the last scanning event, as in the past sometimes they show as 'delivered' when they get to the locker, and sometimes as 'awaiting collection', and if the latter, it can never be updated beyond that because no one mans the lockers to do the final scan event.
When someone signs up for a parcel locker, I believe they have to provide an alternative delivery address, and any AP mail that is not deliverable to the locker gets redirected to that address. I have personally addressed numerous large letters to parcel lockers, and none of the buyers have ever messaged me with any issues with those, but none of them were registered. I dd send a parcel which required SOD, and that went to the locker.
If it's a high value item, maybe paying a little more for a parcel might be possible? A registered large letter would be $5.10, $5.80 or $6.20, depending on the weight, so if a few dollars extra wouldn't hurt, you could send it via the parcel service just to ensure it goes to the locker.
