I like the idea as a measure for potentially getting out of tight situations where you're blocked in by other empires. It does raise the question of breaking immersion, though, as the starting STL engines are based on modern chemical rockets, which are nowhere near powerful enough to achieve anything close to relativistic speeds, which would be necessary to make any interstellar travel in times measured in years rather than millennia. Perhaps it could be used as an event, rather than as a typical travel measure. You can send a colony ship or an expedition fleet off towards the nearest star with engines that are simply designed to continue accelerating to relativistic speeds in a straight line. They'd then move out of your control and have the potential to start a new empire on a habitable world.
Kinda like how the Commonwealth of Man got started.
can be anything from a ramjet to an alcubierra drive