I have the tanto spike and the Persian spike. If you were willing to alter the sheath a bit you could carry a spike with a Spyderco G clip. The clip however would be a bit wider than the sheath, which might look a little odd, but shouldn't matter from a practical aspect.
I love the G-clip. I've already converted a number of knives, from various brands, to G-clip carry. In fact I'm going to order a few more G-clips from Spyderco tomorrow and do the conversion on my spikes. Neck knives are of dubious value , I think, unless you're somewhere you can wear them on the outside of your clothing.