The facility you describe is an interesting one, and one that I would benefit from with my own TinyGPS library. NewSoftSerial and TinyGPS work pretty well together now, but the user still has to regularly poll the serial port and then send whatever characters show up to the TinyGPS object for processing. I've sometimes thought that it would be cool to be able to hook the two objects directly together, telling NewSoftSerial, in effect, look, when a byte arrives, don't stuff it in the RX buffer. Instead, just automatically send it (or several) to a "byte consumer" for processing.
The problem is that this opens up the potential for having interrupt handlers that are so long that the system mysteriously fails. Software serial RX interrupts are already very long by most standards. It certainly wouldn't be hard to implement a facility like the one you describe, but I fear the support issues that arose would be insurmountable. Or perhaps you have a vision that I don't see?