Certainly the method of brainwashing him seemed to make use of his VISOR implants.
Also, we might surmise that it was more difficult to kidnap LaForge than it would have been to catch some other Enterprise officer whose duties took him away from the ship on a regular basis - say, some sort of a researcher who did these trips every month, not just once per season. So the fact that the Romulans did go for LaForge might suggest they had no other option, that only LaForge could be corrupted.
Really, there was no other obvious advantage to using LaForge. He wasn't the officer most likely to come in contact with the Klingon he was supposed to kill. He wasn't the officer least likely to do that, either. He wasn't in a particularly advantageous position to beam Federation hardware down to the rebels, either.
But it seems the brainwashing didn't take away LaForge's initiative or imagination. So his versatility in manipulating the ship and its functions would have remained intact, and would have made him very good at covering his tracks. Which is probably a good skill for a "standby" assassin to possess, until he's called to do his dirty deed.
Timo Saloniemi