NxBRE's Flow Engine is currently under work: I have added a simple backward chaining scheduler that can execute sets in order to produce a specified goal in the rule context. Not all rule bases qualify for it: no construct should exist outside of any set to be usable in this context.
This will allow to support Reaction RuleML as an input, alongside the current proprietary syntaxes, hence to be able to consume the output of Acumen's RuleManager, an excellent tool for BRE related development on the .NET platform.
Stay tuned for the upcoming new release. In the meantime, the most adventurous can already try the backward chaining engine by checking out the latest version out of SVN!