In the meantime, here's a $40 IKEA Erslev rug I stencilled. I'm not sure where I got the stencil -- either Royal Design Studio or Cutting Edge Stencils, I think, but a quick search is not finding it again. I think the reusable stencil cost about $30 too, which would be a better price if I could think of somewhere I might actually reuse it. It'd be nice if there was a local exchange service for used stencils! I probably don't want this same exact pattern somewhere in my house, but I'd totally be willing to trade for many other patterns.
I used a half-can of leftover Farrow & Ball paint from my dining room (Brinjal), which I don't think I'd recommend. It'd clay-based, and it goes on thick and takes a lot to cover solidly. I think you'd probably do better with regular house paint. And you might want to mix it with fabric medium, although that seems like it could be expensive. Others have found painted rugs to be nice and soft to walk on, but mine is emphatically not -- it's quite stiff. That works okay for an outdoor rug, but I wouldn't want it indoors.
Stencilling it took me about an hour of labor-intensive work. Kind of fun, but also painstaking and messy. I wasn't going for a super-solid look -- if I were, it might have taken an extra hour. I was okay with the paint going on unevenly and some of the original white showing through.
I'm reasonably happy with the result, and I might even stencil a rug again someday. But if I were doing it for indoors, I'd want to experiment a bunch first.