The slider should be able to be used to control the throttle. Mapping to it *should* work like any other button (i.e. double clicking the action you want the slider to perform - like say absolute speed - and moving the throttle).

Here's a thread where I describe my config:
http://www.gog.com/en/forum/freespace_series/great_game_but_seems_to_require_4_hands_to_play

I use a Logitech Extreme 3D pro. Like the user above it has a twist handle for rotation/yaw depending on how you set it up. It also has 12 buttons. Here's my organization for them:

Button 1: fire primary
Button 2: fire secondary weapon
Button 3: launch countermeasure
Button 4: cycle secondary weapon bank
Button 5: target ship in reticle
Button 6: cycle forward primary weapon bank
Button 7: target closest attacking ship
Button 8: target target's closest attacker / or target next hostile bomb or bomber - I've used one or the other there
Button 9: Equalize shields
Button 10: Reverse thrust (good for mod Blue Planet: WiH) / or augment forwards shields - again I've used one or the other
Button 11: afterburner
Button 12: match target speed

hat control: viewing angle (default)
throttle control (Rz in game): absolute speed (default)
twist handle: bank (rotation)
y & x axis: pitch and yaw

With this configuration I rarely take my hands off the joystick - pretty much only for alt-J, switch between dual or single fire for secondary weapons (though I often just leave on dual fire for dogfighting), and wingmen commands - which tend to be things that you don't need on a regular basis during the heat of combat or you get used to hitting them very quickly :). However, you should play around and see which commands you use most often. If you don't have many buttons on your joystick you should try grouping the commands you use often during combat in the same place on the keyboard so they are all in easy reach of a single hand.

Another forumer posted his config in that thread:
H2IWclassic has an 8 button joystick:

1 - primary weapon
2 - countermeasure
3 - target closest enemy
4 - secondary weapon
5 - nearest hostile bomb\bomber
6 - transfer energy from laser to shield
7 - cycle primary bank
8 - cycle secondary bank

In this thread (http://www.gog.com/en/forum/freespace_series/joystick_control_scheme) crimecities uses the following setup:

Button 1 (trigger): Fire Primary
Button 2 (missles): Fire Secondary
Button 3: Launch Countermeasures
Button 4: Target Ship in Reticule
Button 5: Target Closest Attacking Ship
Button 6: Target Next Escort Ship
Button 7: Target Nearest Hostile Bomb/Bomber
Button 8: Afterburner
Button 9: Target Next Live Turret
Button 10: Target Next Subsystem
Button 11: Cycle Forward Primary Weapon Bank
Button 12: Cycle Secondary Weapon Bank
Joystick Hat Up: Target Next Closest Hostile Ship
Joystick Hat Down: Target Previous Closest Hostile Ship
Joystick Hat Left: Transfer Energy Laser --> Shield
Joystick Hat Right: Transfer Energy Shield --> Laser
Turn/Yaw Axis: Joystick/Mouse X Axis
Pitch Axis: Joystick/Mouse Y Axis
Bank Axis: Joystick rX Axis
Absolute Throttle Control: Joystick Z Axis

That should give you an idea about what commands and mappings are used most often by people.