It's a tricky one, and something I wondered about quite a lot. To be honest, I went for a couple of full time interviews, and if they went well, mentioned the possibility of p/t or condensed hours at the end of the interview. I was then rejected from both of the jobs I went for, for extremely dubious reasons. An example, 'You're too bright too work in the NHS and I'd worry that you won't fit in'. Oh FGS, pull the other one, translated, 'Now we know you're a mother and therefore a complete PITA, forget it'.
Having said that, you may touch lucky. I've been looking for a p/t post close to home since November 07, and nothing, and I mean nothing, has come up, but I am in the North West, where HR jobs are all in Manchester anyway.
I think it also depends on your level, I see a lot of HR Admin posts that are p/t, but HR Manager posts tend to be f/t.
So, in summary I'd say, it's a risky strategy.
God, what a load of waffle.