Deflector shields usually aren't oriented in such a way as to deflect away from the direction of travel However they are steerable, and (thinking back to TNG and the 'laser incident') they do appear to give protection in none front vectors, since of course a fast moving space object could come from anywhere in a 3D space!
I'd probably give the operator a bonus, with respects to the 'automatic' aspect, but to 'steer' the navigational deflectors so as to deflect something heavy, from a non front facing attack. - bottom line is you're using them for something they are not *quite* intended for!
Beyond this I'd use the rules with respects to ramming and using the tractor to push away objects the size of a shuttle or larger.
The other main problem is how to scale speed. Full impulse is .25 C or 74,948,114.5 m / s - a bullet (optimistically) will travel at about 1,219.2 m/s - so basically compared to something traveling at 'super science speeds, it's gonna be kind of feeble!
In contrast a Rail gun style weapon, will accelerate objects to appreciable fractions of C, so it's fair they'd do decent damage, but then we've discussed this Re: deflector shields!
The other major hurdle I can forsee in the rules is the accuracy - with respects to the technology level you are talking about. Other than 'chaff' (large volume of spray in roughly the right area) how are you going to deal with range: For example, using 21st level analogy technology, you're trying to hit an object which might be travelling hundreds of times faster than a bullet with a... bullet! Again, then scale that up to rail gun style technology, you still have the computational and sensor accuracy issues. As we've discussed, some of the technology with respects to Trek era shields is 'countermeasure' aka fooling the sensors, and frankly Trek era ships could run rings around 21st century level of technology in that regard, which means that they can only fire guns within point blank 'visual' range (using the Icon/Coda meaning of that) - trek ships can fire their phaser beams from ***40,000*** Km away! That said, assuming you're using comparable technologies you also still have the general problems of accuracy: phaser beams are energy beams controlled to the nano-ark by sophisticated computers, and photon torpedoes are well... self guided. How would the equivalent of a mortar shell compare with either: How would you make your launch mechanism accurate enough?! Gah it's a problem when you try to mix infeasible superscience with real-world technology