Found answer after searching MC21 new features -- info copied below to help others who may be looking for this info. It looks like my version of madVR was not new enough, there were no madVR settings for 3D.
I have a number of Blu-ray 3D rips to iso files that mount (with PowerDVD) and play with MC21, but only in 2D. Is there something in MC21 I can change adjust or plug-in that will enable playback of these ISO files in frame-packed 3D? I had another app that played the 3D ISO files for a couple of months, then it stopped working and would not do it any longer... not sure what happened, but that app is now gone.
If not, do MC22 or MC23 play frame-packed 3D Blu-ray ISO files? I don't really need any of the features in the newer versions, but would pay the upgrade fee to get seamless frame-packed 3D playback.
Copied from madVR support:
Media Center 21 introduces support for natively playing 3D Blu-rays and 3D MKV files. This feature will be available in builds 65 and above.
MC21 uses the 3D decoding in LAV Filters and the 3D rendering in madVR to achieve this.
This will allow 3D Blu-rays and 3D MKV files to be played in full 3D on a 3D TV. Only unmodified 3D Blu-rays or MKV files created from 3D Blu-rays with MakeMKV are supported at this time, other 3D file formats, like Side-by-Side or Over-Under are not supported in this initial version, but may be supported later.
To use 3D playback in Media Center, you need to fullfill the requirements listed here, and follow the setup process outlined below.
If you have any questions regarding the setup, please ask them here, and we'll try to adjust the setup guide as appropriate to make it more clear.
3D video can be send to the TV in different ways. The most common and highest quality is using HDMI 1.4 3D support, otherwise known has HDMI "frame-packed". In this mode, the views for both eyes get send to the TV uncompressed and uncompromised.
For setups which do not support "frame-packed" output, madVR offers alternative options, including the well-known "Side-by-Side" (SBS) mode, "Top-and-Botom" (also known as Over-Under), and line/column interleaved.
Note that all non-"frame-packed" modes output the image at a reduced resolution, which might compromise image quality or the 3D effect.
Requirements
Media Center 21 Build 21.0.65 or newer
Red October HQ and a system capable of running it
A 3D TV or Projector and matching glasses
Additional Requirements for frame-packed / native 3D output (does not apply to Side-by-Side output)
HDMI 1.4 and Windows 8.1 or newer
The system to recognize the TV as 3D (Stereoscopic 3D options in the systems Display Settings dialog)
Note that 3D Decoding is currently not hardware accelerated, so a decent CPU is required.
Setup
MC Options -> Video
Video Mode: Set Red October HQ
Enable 3D Blu-ray and MKV 3D decoding
madVR settings
devices -> <your device>
Set device type appropriately
properties -> 3D format: auto (for HDMI frame-packed, select any other modes as appropriate)
rendering -> stereo 3d
enable stereo 3d playback
set madVR to enable and disable 2d/3d mode as appropriate
(only relevant for HDMI frame-packed output)
If madVR is set to automatically enable and disable 3D mode, then simply playing a 3D Blu-ray should switch the TV into 3D mode, otherwise you'll need to switch manually.
Automatic switch is the recommended setup as you can just play a 3D movie without manual intervention, however it has caused issues with a few graphics drivers in the past.
Note that even though we've been working on this feature for a while, its still in a an early stage, and exposure to a large variety of hardware combinations will certainly expose new problems along the way.
Please do report them and we can then try to help resolve them, or make changes in MC, LAV or madVR to improve the playback, if possible.
Known Issues
On some systems, the 3D effect does not engage properly unless the MC UI is briefly shown, or exclusive mode is used in madVR
On systems with Intel GPUs, 3D frame-packed output can have the two eyes showing slightly delayed
Unfortunately there is no universal answer to which hardware works best with 3D output, all vendors (AMD/NVIDIA/Intel) have shown problems along the way.
The development discussion about this feature can be found here: Blu-ray and MKV 3D support [Under Development]