Many thanks for your detailed and helpful advice.
Our COVID-19 workplace risk assessment has highlighted the need to reduce as many touch points as practicably possible and as a result we are considering many options such as using this device.The plan is to out-source the manufacturing of these units to a local engineering company, so unfortunately a fire performance test would not be available.
I've suggested using 2mm thick washers as spacers to reduce the surface area contact of the metal plate, do you think that would be of use? I'll also look at redesigning the plate, with chamfered corners and possibly an additional screw hole in the centre.
Agreed that durability may be an issue on fire doors but it may be that we end up using this device only for non-fire doors such as toilets and use bolts as fixings.
Again, many thanks.