If the beam worked as a device that forced the target ship to match speeds with you, you'd need to stop to halt your target*, which would make you vulnerable. If the beam only affected the target vessel's speed, then as long as its pilot reacts swiftly enough, a fighter/bomber should be able to turn out of the beam, especially if they've got engine kill available (this beam would use CD slot, so you'd need another vessel with actual CD missiles to interfere with engine kill and cruise engines**.)
IF the light fighter was trying to hold the beam over it's target, it would severely restrict it's ability to dodge, so any large ship would be able to easily blast the light fighter***.
This is all conjecture as the game's engine may not actually allow weapons to directly affect another ship's impulse speed. It's an easy enough thing to pseudo-code, but whether it would actually work in Freelancer is something else entirely.
* not entirely unlike when the Police pull a vehicle over to the side of a road ( I know real life comparisons are generally a bad idea when it comes to video games, but I think this one stands)
** beam would possibly need to prevent the target from engaging cruise engines; that or cut the speed in cruise rather heavily.
*** it's entirely possible that transports would probably be able to do this also.
