As basicwiz suggested, use BB's EnvSphere but use an HDR or EXR image on it. I've just set up a test scene, and I have the HDR image plugged into Alt Diffuse via an HSV node with the V value = 8 (that's equivalent to 800% intensity).

Since there's no direct light, there'll be no hard shadows, but there will be accurate soft shadows.

This kind of lighting requires pretty aggressive render settings - note that I've disabled the Irradiance Cache - and it looks like the test render will take hours.

I'll post the results when I can.

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Windows 10 x64 Pro - Intel Xeon E5450 @ 3.00GHz (x2)

PoserPro 11 - Units: Metres

Adobe CC 2017