I've had a couple of cases where I needed a feature, that there had been lots of requests for, in existing software whose development had slowed or stopped. I offered to hire the developer, bounty style, but they weren't interested.
I hired professional programmers to add the feature or make necessary changes to the existing code. I then submitted the code as patches to the original developer, hoping that he would accept the patches and make it so I didn't have to patch and compile everytime there was an update or distro change. My patches were always GPL and there were no restrictions on them, so if the developer didn't like the style or specific implementation, they could use my patch as a starting point or model and change whatever they chose.
In all cases, the developers have not incorporated the patch. In most cases, they have done nothing at all. I'd likely have been better off just buying Windows COTS.
Have their been any updates at all since you submitted your patch? If not and the time period is long enough to believe there never will be, then your best course of action is to fork. As one with enough vested in the project to pay for further development, you are probably in a better position to steward the project than the original developers, who likely have no more use for the program.
If there have been updates, then you have a more sticky position. Most likely, the maintainers considered your patches to be too narrowly applicable at least relative the difficulty required to integrate and maintain them. At that point, you are pretty much stuck re-integrating your patches with each release.
Windows COTS wouldn't necessarily solve your problem either. It just takes away the option to patch your own. If the company is not interested in making the changes you request, there isn't much you can do about it. The exception would be of the commercial software is more popular and better maintained but that's true in the open source world too. If you have a choice between two projects, both of which an do the job with adjustments, you are most likely better off contributing the one that is actively maintained than the one that isn't, even if the required changes are more extensive.