I gave up on these products a long time ago. One of the issues was that your phone/MP3 player would change and render the armband obsolete. The other was that the thing just wouldn't stay put and would end up not on the bicep but somewhere by the elbow, as Seren says. When I was reading the thread, I thought you were going to come up with something like the y-fumble http://www.y-fumble.com/ or the one Trogs posted, which I think do a better job than these armband things. These days I just use a plastic bag to keep it dry and cram it into my shorts pocket. Or go for a very small waist pack. The armbands just don't work for me.
Regarding your product, I had a look at the Kickstarter site and I would like to try to say something positive but I must say, Phil has said everything I would have said so I simply agree with him.
I've not looked into rental suits for several years. The only company I know that used to do them was TriUK. They were fairly basic suits, made by Foor. I'm not sure how much they cost to rent for a season.
A wetsuit can cost anything from £100 to £500 or more to buy, no idea to rent. You have to decide how much you are prepared to spend I guess. EDIT: mind you, I'm not sure many companies hire out top end ones, I suspect there's no margin in it for them.
You'll struggle with free parking right in the middle of Bristol City Centre. The only free parking I can think of is on Brunel Lock Road (google 'free car park Brunel Lock Road' and you'll find it). That's quite a pretty walk along the river to get home after you've been out and it's not too far from the Ibis which Nursey quite liked the look of. However, I'm not sure if you'd get the van under the height bar. There are some single yellow lines on Welsh Back, just round the corner from The Grove, but it's A) if you can get a space there, B) you need to be gone by 0800 the next day - at a guess - and C) if the police know you're in the van, I suspect you'll get moved on. There may be single yellows in Queen Square too, can't think if they've gone now.
As for The Beast, I think someone posted a link to the caravan site on Cumberland Road, which is quite near The Cottage pub (very nice pub). Fat Buddha was going to stay there once for a Bristol do but it was fully booked, so I don't know much about it.