Here's a workaround for using the slider on the throttle for the speedbrakes. With notepad ++ edit the default.lua located in mod/aircraft/f-16/input/f-16/joystick. add this command to line 8:
{down = hotas_commands.THROTTLE_SPEED_BRAKE, up = hotas_commands.THROTTLE_SPEED_BRAKE, cockpit_device_id = devices.HOTAS, value_down = -1.0, value_up = 1.0, name = _('Speed Brake Switch - EXTEND/RETRACT'), category = {_('_Custom'), _('HOTAS')}},
You will see a line added in your controls called Speed Brake Switch EXTEND/RETRACT. Map the slider to it and there you go.