Did quite a bit of head scratching over this last night so please excuse the long reply
I would be almost making this into two infusions, top and bottom if you like. As adding flanges in the normal manner will give you release issues the only thing I could come up with (not sure why it took me so long) was to just make the full size mould 100mm or so wider than the finished chair width. That way you can keep the spiral and hose connectors off the finished laminate.
As you have done I would run the spiral a full 360 degrees around one edge, but have two resin inlets one on the top side of the chair and one on the bottom side (hope that makes sense). Then place a vac outlet directly opposite each of these. By having the wider mould. you can now place these on the laminate rather than on the sides.
This will give you a direct air path for the vacuum preventing the lock off we talked about.
If one side for some reason infuses faster than the other you can clamp that resin feed off until the other catches up. The same goes for the vac outlets.
One thing I would be worried about on the full size version would be release, that's a lot of surface area gripping the mould The thicker you go on the laminate the less flex you will have helping the release. I know this suggestion would mean more work making the mould but if you built in a very slight taper
(across the width) I doubt this would even be noticeable to the naked eye on the finished chair but would greatly help the release.
As for surface finish, soda blasting gives a rather nice matt/frosted finish to carbon. I use it to make chess boards. Mask off the squares with gaffer tape, soda blast, remove the gaffer tape and you have a contrasting gloss/matt chequer board pattern.
A bit of artistic masking and you could end up with some interesting effects.
Hope that makes sense and helps a little, Warren
Carbon Copies Ltd