John: I agree that it would be possible to set a numeric criteria and use your flight recorder like we do for other SUA numbers. I do need to state the issue in different terms...

The class A SUA is effectively set in terms of pressure measurement at nominal 17,500. It doesn't matter that one's flight recorder is reading wrong by +/- 500ft. For SUA, we simply all agree and understand that we are going by a known faulty pressure measurement. In fact, the expectation that it will read wrong by 500 ft at some probability is the reason that it's not an 18,000 ft contest criteria. When we are at the start cylinder, our altimeters have been recently referenced to field elevation; so in that case the measurement is fairly accurate. Not so out on course, 100 miles away late in the day. There will not be a suitable relationship between what is measured and where the ground actually is.

You would have to incorporate an expectable measuring uncertainty into your hard deck. The hard deck would have to be set to avoid the obvious problem that would be created by a false confidence scenario wherein the rules indicate that I'm not outside safety limits so I must be safe enough to keep circling.

You will end up with a hard deck number not very acceptable to very many people. Pilots will prefer to eyeball what is a safe circling height rather than have a faulty measurement dictate when it is not safe according to the rules. To state the problem differently: being scored as landing out due to an unreasonably high "hard deck" when you in fact, make it around without compromising your safety, will seem objectionable to most. I know it would be objectionable to me.

What's more, pilots will ultimately change their circling behavior only minisculely due to a hard deck land out rule -- they still need to get back to the airfield for plenty of good reasons.