I understand the need for lifejackets to be independently certified and packed and sealed in bags etc. on commercial passenger transport flights - but on GA flights I would suggest they only need to be suitable, available and serviceable. The pilot of the flight will need to ensure these 3 requirements are fulfilled. He/she may need to consider what type of lifejacket is the most suitable for the intended flight, and take advice from specialists as necessary, as there are many different types available with different features, such as auto-inflation, strobes, retro-reflective and radar reflective surfaces, and some even include personal locator beacons etc.
Questions to consider
Suitable?
Can it be worn deflated under a harness? You may not be able to put it on in the event of a ditching inside the tight confines of a light aircraft, and putting it on once outside the cockpit may also be difficult if not impossible in a heavy sea.
Does it inflate both manually by pulling a cord or blowing into a mouthpiece, and automatically by contact with water? If casualties may be unconscious in the water then automatic inflation may be required. Is it for a child and therefore does it fit or will it just slide up over their head in the water allowing them to drown?
Available?
Self-explanatory really, either make sure each person is wearing their lifejacket before they get in the aircraft, or ensure there are sufficient carried on-board for each person and they can be easily reached by each person during the flight if necessary.
Serviceable?
Most lifejackets require at least an annual visual inspection and functional check of the inflation mechanisms and air-bladder to make sure they will perform their function if required. Such equipment is also usually 'lifed' i.e. is designed to be used only for a finite time after manufacture such as 5 or 10 years and then destroyed. This is because some of the materials used in their manufacture will naturally deteriorate over time and even if they look OK and the lifejacket inflates - it may not remain intact and therefore may fail to perform its function in a real emergency. Manufacturers should provide information leaflets with their products which describe the inspections and tests that should be performed. Most people can be trained to perform these inspections and tests competently after about 2-3 hours on a relevant training course. However the manufacturers know that many customers have not been trained and would rather let someone else do the checks. They also know they can charge a sum of money for this service so generally do not encourage self-checking by life-jacket owners.
If you hire life-jackets or other survival equipment from reputable suppliers you can be reasonably confident that it has been checked recently and is serviceable, and you should obtain certification to this effect from the supplier.
If you own the equipment you, as the captain of the aircraft, need to make sure that it has been checked and tested and is within its prescribed 'lifetime' before entering the aircraft. If you are not sure you can do this - then you need to get yourself suitably trained. The equipment manufacturer, Royal Yachting Association, or RNLI may be able to offer this training.
When I worked for the National Rivers Authority we did this training and checking in-house for hundreds of our employees who worked on, over and near water on a daily basis. We also maintained appropriate records so we knew which jackets were checked and serviceable and which were not.
Hope this helps.