Well, the issue that the drilling is meant to mitigate is the one we had at our first flat.
The back-boiler is no longer in use and has been disconnected. But where has it been disconnected?
Shortly after we moved in, we set a fire in the back room where the back boiler was.
An hour later, there was a great hissing and spitting from the 'press' (cupboard set into the wall) beside the fireplace.
The pipes from the back boiler had simply been cut here in this cupboard when it had been de-commissioned.
The boiler itself still held it's volume of water, which was now making a bid for freedom!
So we damped the fire down, and the following day I drilled it and drained it.
In your case, I can't see where there's any ongoing source of water, based on your loft investimigations.
Can you even see where the pipes from the back boiler emerge? I imagine they are buried deep in the walls.
I'd be happy to:
1) drill it; and
b) cover it up however you want.
The usual disclaimer applies, and my tame lawyer has already told me 'On your own head, mate!'.