There are universal standalones under development. Major issues include the huge CANbus integration of the platform - the universals will mostly not support all of the current features you ECU does. Another issue is the wide range of requirements for the votlages, frequencies, and peak and hold setups for all the various different injector solenoids. Its a lot more cut and dry in the gas world. The last issue is cost.....
Now if you could get a gas standalone capable of direct injection gas (also under development) - they would have the much more precise timing capabilities required and then its just a matter of building your injector driver circuits to handle the injector of choice.
