I have submitted an enhancement request for you. Feel free to submit any others you have. While this is not hard, I have a lot of other things on my plate, so don't expect a fast response. The project is open source, so you can make a fork and add features - once you are happy with them you can issue a pull request to share your solution.

Since you note you are using SPM, this suggests you have Matlab. Therefore, you can use my Matlab script to create a cluster threshold.