It’s certainly possible; in fact, for some open-source projects, it is the most common way. (A prime example is the Linux project, where the majority of patches is developed for money today; e.g., a hardware company might hire someone to write and upstream a driver for their new product, and other companies are interested in the implementation of certain file system features that they want to use.)
Basically, there are two options:
- You could (try to) commission the MetaBrainz Foundation, which would then have the desired feature implemented by its employees. This has occurred at least once in the past, but I consider it relatively unlikely that MeB currently would accept such an offer, because its developer resources are very limited and more urgently needed for other tasks (see @Yvanz’s reply).
- On the other hand, you could hire a suitable independent developer to implement the feature, and it would likely be accepted just as volunteer contributions are (if of sufficient quality). I’m not aware of this having been done yet, though. Note that, for more complex features that require a deeper understanding of the existing code base, the “developer market” may be quite limited.
Some of the more desirable feature requests will require quite an effort, up to the hundreds of hours of work, and would therefore not be particularly cheap. There are other things, of course, that could be implemented in an hour or less.