Did you find genrule? It’s a generic escape hatch rule that can be used to just run any program. Of course it still needs the input and output dependencies declared and you are responsible for ensuring it is hermetic.
I think part of the issue was that the generated code from the 3rd party tool did not align with the Bazel expectations about code layout (ie. paths relative to WORKSPACE). Likely solvable but not in the amount of time I had available to dedicate to the effort.