Hmmm. The feat doesn't say anything about my allies needing to be within a specific distance (thus I assume no targetting on the backlash is involved). Seeing the barghest, on the other hand, poses a greater problem... but doesn't the description for Suggestion mention him needing to listen instead? I'm a little unsure as to whether I need line of sight for Suggestion to work (I presume I DO have line of effect if he can hear me).[/spoiler]
It's a targeted spell (target line is: one living creature). You have to actually see your target to use spells like this, generally.
Target or Targets
Some spells have a target or targets. You cast these spells on creatures or objects, as defined by the spell itself. You must be able to see or touch the target, and you must specifically choose that target. You do not have to select your target until you finish casting the spell.
I would allow "see" to include blindsight, mindsight, etc, but you don't have any way to see him at all at the moment, AFAIK.
You could delay your action until he is visible, if you're think a fight is about to break out. Or you can do something else.
I think HoV's minions are about to start stabbing people anyway, as he'd told them to ready actions to do so if the pit was closed. I'd missed that, and it might change what you want to do (or not... do whatever you want).