I never did that, simply because I didn't trust the CFI-student to see and avoid all traffic and I wanted two sets of eyes outside. I was PIC for the flight, so I felt my responsibility meant I needed to be looking outside, while playing the "instrument student". The CFI-applicant is going to make mistakes and there's a lot of learning for coordinating with ATC, simulated vectors, appropriate clearances and so on.

I think you've already pointed out that logging that time is not appropriate. It looks like under 61.51(g)(1) you *might* be able to, but you'd need to clarify this, it's not clear. It's a grey area because "safety pilot" under (3) of that section refers to meeting the recently requirements of 61.57(c), which you are obviously not doing. Also, who is the PIC during this time? I don't think you want to go down this road. But if you do, ask the FAA for a legal interpretation. It's not hard and I personally know several people who have gotten the answers back, positive ones at that.