Hi, thanks for posting!
NCrunch doesn't support this extensibility in xunit v1 because it is walled away from it by Gallio. NCrunch relies on Gallio to execute xunit v1 tests on its behalf, and the abstractions enforced by Gallio make the dynamics of this extensibility impossible to implement. You may have some limited success by changing your 'Framework utilisation type for Gallio/Xunit' to 'DynamicAnalysis' (solution-level NCrunch setting), but I would recommend against this as it will severely degrade performance and I am not confident that it will provide a reliable solution.