I've used lots of closed-source "substitute, but just as good for porting" packages over the years. They NEVER work--sure they get the easy 70%, but they all miss the 30% of edge cases that code somehow always manages to trigger. I am not interested in debugging your porting library so you can make more money, thanks--especially if it is likely to piss off a customer. I will control my customer experience as that is my money.
The only exception to this is if you release your code open-source. Then I might adopt it since I can fix a showstopping bug myself if I need to for a customer.
Here's my litmus test if your closed-source package on Unix might be acceptable: Do you support FreeBSD? My team is smart enough to abstract code in such a way that it can run on Linux/FreeBSD/Solaris without killing ourselves. If your team can't do that, it's just going to get in our way.