This is not possible. JACK transport does not support looping.
This is a limitation of JACK and it is mostly intentional: To synchronize transport among many applications, one would have to establish if all apps support looping.
Furthermore JACK transport was designed to accommodate slow-sync clients that may need to read large amount of data from disk. So JACK transport has a state to wait for locates to complete. This makes it unsuitable for seamless-looping. Last but not least, the state is bound by block-size. It would have been possible to specify explicit loop points to applications, but that likewise has synchronization problems.
The general consensus is that JACK was superseded by LV2 for cases like this.