The question is always why anyone would sustain over flexion at all (aka closed throat latch let alone HELD btv (with torqued curb). It goes against the horse actively seeking the hand and easily chewing fdo.
There really is little difference whether it is 30 seconds or an hour (seen in warm ups before 'the 10 min. rule'). WHY would the rider HOLD the horse closed? #$& happens when schooling, horse curls over, ok half halt and carry on. It's rather like asking the question how much beating of the bf/gf is ok?
For the horse it is an effect on the carrying mechanism, the bars, the balance, etc. Why use the neck against the horse rather than WITH the nature of the horse?
A horse that is very supple in an uphill posture will not easily go ldr because it is uphill. It will however chew fdo quite easily because it trusts the hand to open its posture and step into that connection (without becoming over tempo). Lightness in the mouth is a result of a mobile jaw and self carriage, not because it is posed or too compressed.