For now the only option is to hire a service provider to do the coding for you or to do the coding by yourself. In case of hiring a service provider there is not guarantee that such development will be contributed to the project.
See this example.
In this case Alex creates an issue that he is willing to pay for and a developer creates the PR assigning himself to work on it.
That said, the maintainer/petitioner could end not paying or, like Sergi said, there is no guarantee of merging the developers work into the main branch of the project.
A common case that could happen would be if the petitioner requests a new feature while being on the LTS 6.0 but new features are only implemented for the (current) 6.7 branch (6.8 to be).
I think maybe Tryton could provide a bounty wall on the forum once/if the discussion on non-official modules settles. The petitioner would create the issue and the bounty post.