Sailrite has a couple videos about sewing webbing, with stress testing, etc.
Another DIY trick might be to line a tubular poly/nylon web with a run of Dyneema...This is how APS makes their's
No, Not ever. Dynemma is acceptable for jacklines, but NOT for tethers.
The ISAF and ISO standards include a drop test and require some level of shock absorption. There are reasons no commercial tether is made of anything other than nylon.
Try this test. Make a tether out of something non-stretch like Dyneema, tie it to a tree, gather up some slack, and take a run at it. If you leave more than 2 feet of slack you are going to get bruises, and with 4 feet of slack, expect a back injury or cracked ribs.
Tethers must ALWAYS have some stretch.
There are two VITAL shortcomings with the Sailrite testing:
1. They tested polyester webbing, which does not stretch. Sewing a material that stretches, like nylon, requires a different method. It's like sewing elastic. A box stitch is not used for this application, so the formula and testing is not valid.
2. Nylon is always bar tacked. Look at ANY climbing equipment. There are engineering reasons for this that have NOTHING to do with economy.
(I used to be in the industry and have tested sewn nylon--I'm not guessing or reading off the net)
They very wisely stopped selling the harness and tether kits. They did not meet any of the required standards. They are sailmakers, not structural equipment fabricators.