As discussed at support.sourcetreeapp.com/discussions/suggestions/181-custom-external-actions it should be possible to launch external scripts against arbitrary files.

This enables users such as myself who are developing in non-native environments to automate their workflow (eg: developing software for arbitrary non-native platforms, using virtual machine instances or emulators to perform code deployment and testing).

This is a HUGELY IMPORTANT feature for me and I have patiently waited months for its implementation with no news so far...