I loved the near-witch scarf so much that I pulled out two brand-new skeins of Malabrigo and gave it another go.
I used, as far as I can tell, the same length and width of warp, but the dimensions came out a bit different.
The non-reclaimed warp gave me a 77-inch scarf that was only 7.5 inches across. The near-witch warp ended up being 70.5 inches long and 8 inches across.
I paid more attention and was generally pretty fussy about the edges on the non-reclaimed scarf. I might have ended up pulling the weft a little bit tighter in doing that.
The non-reclaimed scarf was longer because I made a real effort to reduce the amount of loom waste. When tying onto the beam, I sometimes end up with really long ends. This isn't really a problem, except that you have big knots to deal with and, of course, you have less room to weave. So, I wound the warp until it was quite a bit shorter than usual before tying on. I figured, if I really hadn't left myself enough room, I could always unwind it a little bit.
This turned into an interesting experiment. Conclusion: don't be afraid to reuse yarn, but you might end up with a slightly different result.