There have been several recent attempts to include duplication-based on-line testability in behaviourally synthesized designs. In this paper, on-line testability is considered within the optimisation process of iterative, cost function-driven high-level synthesis, such that on-line testing resources are inserted automatically without any modification of the source HDL code. This involves the introduction of a metric for on-line testability. A variation of duplication testing (namely inversion testing) is also used, providing the system with an additional degree of freedom towards minimising hardware overheads associated with test resource insertion. Considering on-line testability within the synthesis process facilitates fast and efficient design space exploration, resulting in a versatile high-level synthesis process, capable of producing alternative realisations according to the designer?s directions.