On the topic of an air bag suspension--I been highly into the idea and been doing research for a few months now. The fact of the matter is, there indeed isn't a bolt on kit for the STS, HOWEVER; I finally came across a company that may have an anwser for us.
I been exchanging emails with Tom Parkinson from airbagit.com--they specificly design each of their kits to match factory top and lower mounts for the struts/shocks/springs. Basically; they should be capable to build any kit for any car.
After taking some pics and measurments of the OEM suspension setup on my 05 STS V6 equipped with only "soft ride"--it should be a pretty straight forward setup to install the air suspension.
Here is some info he sent me in an email--very informative IMO:
" This kit here is a great start for your vehicle. It comes with everything you need to get started. There are many ways that we can put these kits together and if you call me here at the shop I can tailor make this kit to suit your needs and more importantly your budget. I would love to assist you in your build. If you have any questions at all please do not hesitate to ask. I look forward to hearing from you soon. Have a great day.
Kit to start Strut / Coil
http://www.airbagit.com/v/vspfiles/Pages1/FactoryCoil.html
Upgrade options
http://www.airbagit.com/SearchResults.asp?Search=upgrade-
Knowledge base
http://www.airbagit.com/v/vspfiles/kb_html/index.html
Tom"
I will state, however; it's been a week since I've heard from him--I hope to hear back soon.
Another option, may be using AirLift's universal kits but you will in fact have to make your own top and lower mounts.