What are some of the most feasible or recommended ways for getting companies/commercial investors involved in an open source project?
Show them evidence of other open source software companies making money such as MySQL etc, all investors want to see that they can make some money.
http://news.cnet.com/2100-7344_3-5216387.html
http://www.infoworld.com/articles/su/xml/01/10/15/011015suopen.html
http://xavisys.com/2008/05/how-to-profit-with-the-open-source-community/
The simplest and most powerful way to do this is to prove to them that there is an economic incentive to contribute their resources.
The power of a sufficiently interested community can be quite immense. The man-hours invested in some open-source projects are almost immeasurably large, and it shows. Wouldn't it be great to have that power helping to improve the products they depend on?
Work to remove some of the stigma from Open Source. Businesses often assume that open source code is like old shareware, created by hobbyists. There's also the stink around GPL being a viral license (requires you to open your code if you use their open code).