"Other programs that render with HDRI have both the reflections and IBL also and it's not unusual to control these seperately."

...take a look at Vue 5 Infinite (it might be the same in standard Vue 5 also)-- you can use HDR files in that manner...

RE: True Ambience- if you have a great scene and patience- you might consider rendering it TWO times- one with HDR and one with TA, and merging them in Photoshop... this would be my suggested "work-around" until B6 aquires more lighting controls... I use this "multi-pass" technique sometimes with V5i- it seems faster to me than cranking in HUGE amounts of lighting effects in one single render- plus you have more control over the effects "in post"...