Not permitting the use of snorkel masks is, in our opinion, quite correct for a number of reasons, not the least of which are the lack of training available, no product standardisation and the potential for CO2 build up in the mask.
The device you are describing is less of an issue although, being a snorkel, there remains the ever present problem of expelling water from the tube and that is a technique that does have to be taught and learned.
We have been involved in the decision making process in a couple of these where someone with a disability needs to use this type of device to allow them to continue to swim and the outcome has always been positive. We also came across it where a swimmer has a tracheostomy following throat surgery and needs a snorkel style device in order to breathe safely in the water.
We have discussed this and we are in agreement that this is to be encouraged for the benefit of the swimmer but that, where possible, use should be restricted to a lane so that there is a reduced potential for a collision that could cause the swimmer to lose the snorkel. It also has the advantage that, and not suggesting that the swimmer is a higher risk, actual monitoring by the lifeguards is easier.
One of the team was involved, as the Crown expert, in a related drowning where this approach was used with an adult male swimmer who was receiving treatment post from a spinal injury and similar to the circumstances you describe, was unable to lift or turn his head at that stage. In this case, the lifeguards were distracted by unnecessary chatting with two other staff members during which time, the swimmer became incapacitated and was not observed sinking to the pool basin floor.
Providing that lifeguards are attentive and supportive of the swimmer and understand why the snorkel is necessary, taking into account the above, we would support its use.