I'd suggest definitely putting OS projects on your cv, if they're more relevant skills wise personally I'd be happy to see them ahead of your career history, probably with a quick explanation as to why above them.
What sort of jobs are you going for? I do occasionally get cvs from people with a lot of experience in other languages, and some in Java which I'm hiring for. The main problem is they're being put forward for positions based on past experience in other languages. Whilst a lot of what developers do is transferrable I have to measure them against other applicants with more relevant experience so often they're too expensive vs the competition. If they are applying for a more junior role to balance this out there's then a risk to me they won't be happy with the salary I can offer them and/or will return to their previous work.
Given the above I'd suggest tacking the risk by explaining you're trying to move your career into a new language hence applying for the role. This may also feed the ego of whoever you're applying to that they're working in a desirable language. If you are willing to take a slight step down based on moving away from an area of expertise you can then differentiate yourself against other applicants based on your broader experience.
Your third paragraph sums up the reason for the problem the original post described. Someone has software engineering experience in Foo Language, but wants to get a chance to work professionally with Blah Language for whatever reason. Obviously, his years of Foo programming aren't completely transferrable, and you point out that he's not going to compete for a role with someone who's been a Blah programmer all along. However, if he recognizes his lack of experience with Blah and applies for a junior or even entry-level position, you say that there's a risk he won't be happy with the salary and will skip town.
Your solution hinges on people accepting OS, hobby, academic or otherwise "extracurricular" experience as meaningful. I have not found that to be the case, and the other post someone alluded to described exactly that. I'm not accusing you of causing this problem, but you've highlighted it so perfectly that I think it warrants a little bit of exploration.
I'm going to make a controversial blanket statement and say that any company which doesn't consider extracurricular experience to be meaningful is probably not a company you want to work for. The reason being that side projects, open source experience, etc. are all indicators of passion. A company which doesn't appreciate that experience is a company which either doesn't understand or doesn't value passion in its employees.
I agree completely and we've made hires based more on the strength of peoples passion for programming than a specific skill set. This works excellently I think for junior or mid level positions beyond that though I think you really are looking for specific expertise as opposed to raw potential.
I'd suggest definitely putting OS projects on your cv, if they're more relevant skills wise personally I'd be happy to see them ahead of your career history, probably with a quick explanation as to why above them.
What sort of jobs are you going for? I do occasionally get cvs from people with a lot of experience in other languages, and some in Java which I'm hiring for. The main problem is they're being put forward for positions based on past experience in other languages. Whilst a lot of what developers do is transferrable I have to measure them against other applicants with more relevant experience so often they're too expensive vs the competition. If they are applying for a more junior role to balance this out there's then a risk to me they won't be happy with the salary I can offer them and/or will return to their previous work.
Given the above I'd suggest tacking the risk by explaining you're trying to move your career into a new language hence applying for the role. This may also feed the ego of whoever you're applying to that they're working in a desirable language. If you are willing to take a slight step down based on moving away from an area of expertise you can then differentiate yourself against other applicants based on your broader experience.
HTH