In reply to valjean:
> re: using belay device in guide mode
>
> easier than using a prussic but not by much. requires you to pull up rope through the device. annoying.
Thanks for all the tips in this thread - things that worked out in the end:
1. top: prusik to harness, lower: prusik as foot loop (~30cm loop when on rope). Didn't need any slings at all for this, and works quite nicely.
2. top: prusik to sling for foot loop, lower: belay device in guide mode. This also works nicely; maybe a little quicker than 2 knots, but also a little more tiring? For the foot loop, needed ~90cm sling. Switching over for descent is also really simple with this setup, but not sure how important that would be in real life.
Do people tend to use backups or safety knots? Hanging directly off a prussik feels quite weird, and so does trusting a belay device no-hands.