Audiowerkstatt has launched midi-clock-shifter, a device that allows you to move your MIDI-slave as many MIDI-clock-ticks as you want forward or backward, while running in synchronisation to the MIDI-master.
A tool, for on-the-fly correction of a MIDI-slave, that is not reliable or to push a loop, that is not recorded in a rhythm, to that rhythm. And a tool to have fun with playing around with two sequencers and do things like e.g. going slightly forward and backward with one of them.
You can set whether only the MIDI-clock and MMC commands (start, stop, continue) are sent or the MIDI-data, received at the MIDI-in, will be mixed with that.
The midi-clock-shifter is available for purchase for 149 EUR.