How to find PhD students interested in 6-months paid exchange visits to work on a supervised project? I am a junior research fellow with a bit of a luxury problem. I am in a field with many good, open problems. I have a grant which could easily support salary for a couple of exchange PhD students for say, six months, to solve some of those under my supervision - but how do I find those?

I have tried some of the obvious channels, asking network, including old supervisor. Asking around at conferences. Most senior people I talk to agree it would be a good idea - but I can't seem to get any good people to bite.

I am asking advise from people who have been in a similar situation - also students who have accepted such offers, what made you do it? What made you hesitate? Is the prospects of 6 months of relocation off-putting? Are you afraid what your supervisor might think? And for more senior people, whose students I would potentially snatch up: Would you recommend a good student to do something like this? Would you be annoyed that I "borrowed" your good student, just after you taught them all the difficult parts?
Your post has hit on the problem: you want some PhD students to come work with you for six months. This is a bit too long for a summer internship-like posting, but not really long enough to justify the work required for what would amount to a relocation. It's like a long temp assignment. 

Perhaps you should be looking at trying to support some students who are just finished with their PhD's—or have just handed in their theses and are figuring out what to do next—at nearby universities. Or trying to arrange the funding to support a postdoc for a year.