Anyone here do in-house registry/float pool/PRN?

I'm thinking about doing that as a second job, but I'm not exactly sure what it's like.

I mean, I work with IHR/float pool nurses all the time, but I was curious as to some of the details.

In particular, what units they expect you to work. I'm concerned about signing up for float pool and getting assigned to a unit I hate...like psych...or to one which I don't have any experience...like L&D...or both, like Peds.

Though this option may not work out if the time requirements are as strict as at my hospital, which requires a minimum of once a week.
I bet it varies from hospital to hospital. Why don't you ask some of your co-workers? That is how I got the low down before signing up for it. At my place I can turn down any assignment.
I would say that most hospitals will interview you and ask you what your area of specialization is, and put you on those floors. I have a telemetry background, so when I started as a registry nurse I could work med-surg, telemetry, but also oncology as well, without administering chemotherapy, of course. I don't have experience in peds or l+d so they didn't put me on those floors.
In one facility where I worked, they floated me pretty much anywhere but OR! But I wasn't expected to take assignments in ICU/OB/ER, just be an extra set of hands. Now in my current hospital, it's just the medical/tele floors. It really can vary a lot; you really would need to find out what is current practice in your area
PRN. I officially belong to the budget of one psych unit, but can float to a variety of units/programs. In my facility, the assumption is that if you are hired as a PRN you are capable (or better be) of floating to any of the psych units. So I see a lot of different places. I actually spend most of my time working on a second unit that pretty much has considered me their own...though they won't dare steal me from the first unit since that unit's director has already threatened to wallop them with a big stick if they even try it