I did have a look at omni-wheels, but the only commercial one with a 125mm diameter had rollers that looked like quite a hard plastic, and I didn't want to get it shipped from Australia if it wasn't going to work. I'd probably need a custom-made omni-wheel but there's too much faffing about involved in making one when there's no guarantee that it would definitely do the job.

Will play about with a few different tread materials and see what produces some decent results.