The bulk of muzzle safety violations I've seen are not stage design related - although I have seen targets placed too close to the line and a shooter DQ'd because they crossed the 90 line. I would characterize this as being in the small minority of cases.
The 90 DQs that I've seen are:
1) during a reload - engaging targets "cross court" and executing a reload (poor muzzle discipline) -Beyond 90?
2) during a reload - bad form and turned slightly to the left (poor fundamentals) -Beyond 90?
3) during "unload and show clear" pointing a firearm directly upwards (poor fundamentals) -Don't you mean beyond directly up? Directly up would be 90, last I read we had to be past it.
4) movement around a barricade (poor fundamentals/muzzle discipline or awareness) -I'll assume this is breaking 90, seen it on port too- what goes in must come out...
5) forward movement and re-engaging a bypassed target (poor awareness) -Breaking 90, seen it.
6) engaging a target from a port that should not have been engaged from the port (poor awareness)-I'll assume this is for engaging steel at unsafe distances...So yes- DQ. But what was the match Director doing in creating it in the first place and more importantly what was the Range Master doing- it's their job to ensure this can't happen.
7) lateral movement on a stage - pointing the firearm directly in line with direction of travel vs. pointing downrange (poor muzzle discipline/awareness) - again I'll assume that the direction of travel is beyond 90 degrees.
#6 is also a design issue, but I don't think you should have to place a vision barrier behind every set of targets to prevent shooters from looking at them as they move forward.
You have a bit of an issue in IPSC if you accept the notion of shooting the targets "as visible" - Comstock- and then set up a stage that intentionally removes that ability- especially with a DQable offence- just my opinion.
Also, I can't help but notice how you focused on "at 90"...The rule book the last I read was beyond 90.
Not only do we have a lot of new shooters but we also have (in my opinion) some over savvy RO's who interpret the rules in an overly harsh way at times...
Some rules are entirely dependant on your view point (attitude) and point of view (location).
-You must break 90 (at 90 is not breaking it).
-You must have your finger in the trigger ( the fact that a guy has a bent finger and it's not high on the slide doesn't necessarily mean it was in the trigger...).
-You must take you finger out during a reload- unless your on target ( I hate this rule- was it?...).
-You must have your finger out while changing position or moving. (I give this one to these same RO's- the book is clear. But some RO's are looser with it than others which can cause problems on occasion).
This may come off as ripping the RO's (I'm one) but I'm not in my opinion - or at least the majority of them...
The sport asks a lot of it's competitors and it's officials- and it should.
Shooters are not always perfect, same is true in my opinion on the other end.
I've seen some RO's that literally get off on DQ'ing competitors...
That said, some RO's are a little inexperienced at making any call...
So maybe it all cancels out.