AFAIK, most of that's actually doable. The only one I can think of that poses real problems is the backwards thing, not because the engine doesn't support it, but because I'm not sure if a player ship can be made to.
I could see how sliding/trichording would be do-able, since Shivan ships exhibit that capability in-game, and there's also the presence of the Descent-style physics cheat code. Getting it assigned to my joystick's hat switch seems like it could be another story, though.

For anyone who doesn't know, the trichord speed boost occurs when you're chording (sliding) in more than one direction and tilting your ship in the direction of travel. It allows you to move at about 1.4 times the normal speed when bichording and about 1.7 when trichording. It was originally a bug in the first Descent's code but, after being widely used by pilots as an essential strategy, it was included in the two subsequent games. I can't really see how something like that could be pulled off in this engine, though my knowledge of modding is admittedly basic. It's definitely not something that would be necessary, just a nice touch if at all possible.
Reverse thrusting at the same speed as forward would obviously be heavily preferred, if at all possible, since it's an integral part of the whole 6DOF gameplay style. It sounds as though it might be too involved, though, and if it is, I'd be willing to give it up for the sake of a Descent mod. Nuke, if you do manage to get this thing rolling, I have a whole forum's worth of people who'd be thrilled to beta-test it.

karajorma, that's a pretty cool idea about switching ship models.

If this were to go through, there would be only four types of playable ship models: the Pyro-GX from D1/2, and the Phoenix, Magnum, and Pyro-GL from D3. (The Black Pyro in D3 is basically a version of the Pyro-GX.) Add to that any robot models that would be included, and the rest would be left for multiple ship models. The only issue I could see would be the ability to switch weapons at will; in all three Descent games, this was done by pressing the 1-0 keys once or twice. Would SEXPs or scripting be able to handle something like that?