I don't believe actors were ever allowed to write episode scripts. There are one or two exceptions at most across all of Star Trek, that being Robert Picardo, who has a writing credit(but only a story credit) on one episode of Voyager. There might be one more in DS9(Andrew Robinson?), but I don't remember. On all the shows, however(including ENT), the actors were allowed and welcome to make suggestions and get scripts changed when they had an idea, or were unhappy with something.
Directing episodes requires a huge commitment from the actor who is interested, on top of an already huge commitment as an actor. I know in Voyager, Garret Wang had been asking to direct an episode, but wasn't willing to commit to all the extra hours, so he was never given the opportunity, while many of his co-stars were. So it could just be that no one on the Enterprise cast was interested in directing and/or willing to make the immense commitment.
Frakes, Dawson, and McNeal were so committed to directing that they gave up acting altogether, and have gone on to be very successful TV directors(particularly Dawson)