Has anyone tried these alternatives to rim tape on a tandem? I've been given a set of 40 and not sure whether I should give them a go.

Hi Jay!
Yup! Got them on our ariZona tandem wheels and liked 'em well enough to put'em on my single bike too.
As stated they fit perfectly in the 8mm webhole of Velocity rims and rims with same size web hole opening. On tandem have Velocity Aeohead rims and they just pop into each hole perfectly.
On the single bike, run Topolino wheels and the web hole is a tiny bit too large for the Veloplugs to fit in snugly, and stay in. Solution: tiny bit of duct tape over each Veloplug.
A great bonus with these plugs is, that it gives more room for the inner tube (really!) and makes tire removal/installation easier. Had a flat on rear of tandem and was able to remove the rear tire with 'bare hands' only: no tire tool needed (yeah, I've got *strong thumbs!*). I can usually get tire back on after a flat without tire tools, but the Veloplugs even made installation easier.
So install them and give a special 'thank you' for the person that gave them Veloplugs!
Pedal on TWOgether!
Rudy and Kay/zonatandem

I got them with my new Velocity Wheels and they seemed to fit very well and make a lot of sense. I think there is also a slight weight saving over rim tape for the Weight Weenies out there. But being a traditionalist I went back to the tape and still have the plugs sitting in a bag. Maybe if I have to replace the tape for some reason I'll give them a try.