To date there has been no official documentation (from hand ascender manufacturers at least) endorsing the use of top holes on these devices for life support connection.
Jords Climbing Corner have questioned whether the practice is now officially accepted by Petzl with their latest news release titled "Self-Belay: one single rope with two ascenders".
According to Treetools research Petzl are indeed the first hand ascender manufacturer to indicate that top holes on the device are rated for human life support… albeit in very specific configurations.
Tree climbers seem to automatically assume single rope connection means single line access as used in tree work - typically utilizing the 'frog' system with hand ascender, Croll and Pantin but that is NOT the situation described here by Petzl.
If you read the first paragraph in the article, boxed in yellowy orange, Petzl clearly state: "Ascending on a rope (they say caving, big wall etc but you can also read tree climbing) is not addressed here".
The Petzl article discusses various techniques for self-belay on a fixed single line. The climber has his feet on the ground (meaning rock face), and the belay line is for back-up purposes only. The trialling ascent devices are configured to 'catch' the climber in the event of a fall. Load is only applied when the climber is 'at rest' or he falls.
Single line ascent in tree work is quite different and utilizes a completely different set of mechanics.
In single line access the tree climbers weight (load) is applied passively onto one (or two) of the ascending devices at all times as they ascend up the access line.
Ideally, each connection point is backed up by clamping onto the rope independently to each other - that is, not at the same time. While one device is connected the other is moving and so on.
Tree climbers have sought clarification on the top-hole-hand-ascender debate so that a second (backing up) connector could be attached via the top holes of the hand ascender and be pushed ahead of the device during ascent, circumventing or simplifying the two points of connection rule - in the 'frog' system the two points of contact would be the hand ascender and Croll or similar.
The finer details of single line ascent configurations are too varied and complex to be covered adequately in this blog post so we'll stop right there.
While Petzl may have indicated acceptability for life support connection on the top holes of their hand ascender (in certain situations) it's very doubtful they mean this practice is endorsed for single line access as used in tree work.
We'd be interested in your comments.
Thanks to Jords Climbing Corner for this lead