However today I bestirred myself to actually do SOMETHING, and that something is shown below.
There was a certain amount of clipping and filing to do on the saddlecloths of the Black Hat figures, and I also had to trim around the reins and saddle on the gashants, but all of it was quite simple. The most difficult is the pinning of the rider to his mount, but I imagine that's just me. After an hour's work for the first four, it's looking like a good fit and I'm glad I went down this route.
They still need some filling with green stuff around the edges of the saddlecloths, but they're almost ready for the painting queue.
So a big thumbs up from me for the marriage of Black Hat riders and Highlander gashants.
EDIT 15 May 2014: I should also add that Mike Lewis at Black Hat was happy to send me packs of eight riders (without Octosaurs), so this is looking better cost-wise too!