Yeah, that's right. When you're setting up a FBL unit downstream of the Pixhawk treat the PIxhawk like you would treat the settings in your radio going to a FBL unit, as far as the SERVOx_ parameters. The RC IN is only to calibrate the RC so it matches what your radio does.
We got rid of some of those H_ specific servo adjusting parameters with a change @bnsgeyer made. However, that change is not merged to master yet, much less in the stable release. But it is in my custom helicopter builds:
When you set up a downstream FBL unit set the VFF for pitch, roll and yaw to 0.5. Set all the ATC_RAT_ PID's to zero. Set all the limits for collective pitch (H_COL_MIN/MID/MAX to full travel (1000 min, 1500 mid, 2000 max). The swash ring won't matter because you set your H_SWASH_TYPE to 1, which is an H1 with straight output - no CCPM mixing takes place in the Pixhawk - that's done in the FBL unit. Set H_FLYBAR_MODE to 1, which gives you straight-thru control to the FBL unit in Acro flight mode. Then set up the head limits, swash ring, pitch limits, all the PID's, etc. in the FBL unit. From that point forward setup of the heli is whatever your FBL unit uses for setup, and set the FBL unit to rate-only control mode, don't use attitude control in it.
The only oddity is that ArduPilot puts out its collective pitch signal on SERVO3, and the throttle on SERVO8. (same on the RC, which which makes using all the traditional throttle hold, throttle/pitch curves, etc. hard to use in your radio). Since the radio is going to be WAAAY more advanced than what ArduPilot has for throttle handling, use a Heli setup in your RC with a straght H1 swash and collective pitch. Hopefully your RC radio then will allow mixing of the collective pitch signal (Channel 3) to the throttle signal (Channel 8). I've been meaning to do a video to show folks how to "hack" this phenomenon in the RC so you got a traditional throttle hold and Idleups.
You enable the BRD_SBUSOUT by setting the frequency of the S.Bus signal in ArduPIlot The only connection from the Pixhawk to the FBL unit is from the SBUS OUT port on the Pixhawk to the S.Bus in on the FBL unit. All the servos hook to the FBL unit as they normally would. Your FBL unit will have to support S.Bus mapping so you can map the FBL unit's throttle signal to Channel 8 and it's collective pitch signal to Channel 3. Otherwise all FBL units use Channel 3 for throttle and Channel 6 for collective pitch.
So it's a fairly simple setup on the ArduPilot side, takes a couple hacks in the RC for using throttle/pitch curves and idleups, all the tuning and setup is done in the FBL unit. The Pixhawk provides navigation, stabilization by merely providing attitude command and passing pilot control inputs to the FBL unit, the FBL unit handles the rest. It is actually a quite popular setup for a lot of users because modern FBL units are considerably more advanced than what ArduPilot is for helicopters.