No, a siphon wouldn't work. The entire planet is surrounded by a vacuum, but its atmosphere stubbonly refuses to be sucked away.
To get the overabundance of CO2 off the planet (and maybe export it to elsewhere in the system, the Moon, for instance, or Mars) you would need to lift the atmosphere physically off the planet. Some sort of dynamic ring system
http://en.wikipedia.org/wiki/Orbital_ring
would probably be the best bet - we are talking about mega-engineering here, but it is a big job, so you need to think big.
Hey- there is lots of carbon in the atmosphere- why not use carbon in the construction of the ring? Carbon nanotubes, Aggregated diamond nanorods, diamond itself, graphene; all materials which might be useful for constructing these megastructures.
The superrotating atmosphere itself could be tapped to supply the rings with power (these devices are active, not passive structures, and require power to stay in the sky; even more if they are used to lift material).
Paul Birch gave some ideas on lifting mass out of Venus' atmosphere in a paper written for the JBIS, available here
http://www.paulbirch.net/
Actively extracting atmosphere from the planet may also help to speed up the cooling process; once in space the extracted mass could cool down by radiation.