Many of the directv receivers dont have any stickers on them. Mine dont. Some also had security torx screws, although that was sort of dumb because you can buy a set of torx security bits at sears for $5.
Directv specifies something about not 'tampering' with the box. I guess by definition, opening it and replacing the drive is 'tampering', but I think they're more concerned with anything that creates a theft of signal/service opportunity.
The only fee schedule they've published include the fee for not returning the box or returning it in a non-operating state. Do also note that they cleverly terminate the protection plan prior to your shipping the box back to them, so you cant say you have the PP if they receive a non working box and try to charge you for it.
Perhaps stuff like that is why they have the poor BBB rating and dropping customer service ranking.
So all they could do is if they detected that you had 'tampered' with the box, they could file a civil suit on you for non performance of the contract. They'd have to prove the 'tampering', establish a basis for damages as a result of your tampering, etc, etc. In short: not worth it.
All that having been said, the receiver logs at boot time record the manufacturer, model and serial number of every drive in the box and where its attached (internal or external), and those logs are sent up to the directv mother ship periodically. So they could easily filter those logs for any disk drive models/sizes that are mounted internally and not ones they installed.
The HR24 looks particularly problematic since internal pictures of the box seem to show that the disk drive is encased in some sort of black plastic box, and has a number of wires attached to that box. My guess from looking at it is that you might not have much of a chance of removing that disk and replacing it with another without leaving some evidence.
They also may have made a point of putting a 'warranty void' sticker on every HR24 while they havent been so consistent in the past. And it may be that they're using the newer type of security sticker that leaves a permanent mark on the box saying 'removed' or 'voided' when the sticker is peeled off.
My guess is if you put a disk in there, dont break anything, replace the original drive and the unit shows no signs of physical damage and works properly when directv gets it back, you wouldnt have much of an issue.