Rhyb - we have taken DS in a couple of times in the early days and he sort of knew there was a baby in the big plastic box LOL but was more interested in all the noise & activity of the NICU and just wanted to get down & run around. We show him pictures of DD and he recognises her as a baby, and we talk about her all the time at home & use her name etc. so I think he might be catching on a bit. Like most older siblings though I think he'll get a surprise when life changes once she's home.
Our friends found that their youngest did really well when they brought home their baby. it seemed that with all the preparation etc of the baby being in NICU etc seemed to give him time to adjust (and i reckon that having his mum back full-time also helped...). so could be the case for you guys too!

Awesome news on the open cot!! that must be so cool and less of a physical barrier between the two of you must be something special!