There are sections in the L6 Link Connectivity Guide that indicate you can use Channel B to get the modeled sound straight from the amp. It does indicate that the HD FX and Reverb will be applied, but you could turn those off.
From the L6 Link Connectivity Guide (HD500 page 3-8):
When the POD HD Amp Model is “Off” (or set to “None”), all respective DT
amps switch to Channel B. The controls do not operate in tandem between the
POD HD Amp Model & DT amp Channel B. When in Channel B, the DT amp
behaves exactly as it does in stand alone mode, using HD modeling technology for
the preamp and tonestack.
The section for HD300/HD400 seems clearer to me (page 2-6):
DT Series Amp - Channel B
Whenever your POD HD Amp/Preamp Model is “Off,” the DT Series amp is automatically
switched to Channel B. The Amp tone & settings are determined by the amplifier’s
Channel B knob settings only, and the FX are derived from POD HD. You can think of
this Channel B configuration as your POD HD acting as a fully featured multi-FX device,
with the DT amp internally providing all the Preamp + power amp aspects of the sound!
On second thought, I guess this really isn't an option because you'd get only 4 voicing, if the POD HD allowed you to switch between the Channel B voicings.