I think the biggest problem is- that the cost of nylon is so high- + shipping nylon to crafter, and then a bit of labor to turn the pony tail into a clip, and then ship out to you- well, you could kinda already do that just by ordering from one of the hair people.
The cost of adding a "middle man" would mean the tail would be more like $10 and in that case, many would prefer to just find a donor tail on ebay.
The other problem is that matching is pretty hard- even though most of the hair sites have suggested matches, it is far from a perfect system and as you know, a lot of the hair can fade a bit- and there are also some natural variations between nylon batches- so without having the actual pony in front of you along with the actual hair- it would be impossible to match perfectly.
Thinking about it - it might actually just be easier to see if RestoreDoll would accept you mailing your pony right to her, for a perfect match. I wonder if she'd do that? Probably too busy but it's worth a shot...