I've just had a look through the options in MC18, and it seems that this is not currently possible as there's no way to output unscaled video - aspect ratio correction is always being performed. (MPC-HC has the option to disable it)
This means that even if you were to output a 720x480/576 signal from the PC for DVD playback, the image is going to be scaled in MC. (DVDs are encoded as 720x480/576 and then stretched to either 4:3 or 16:9)
Unless you are watching video type content and not films, you probably don't want to output an interlaced signal from the PC, and are better off having madVR handle it, as it will bypass deinterlacing altogether if the disc is progressive, and you can force film-type deinterlacing if it is not, to avoid cadence detection problems for content that is interlaced. (my experience with the Radiance had it dropping out of film mode too often with PAL content)
I would also suggest that you perform the scaling inside the PC with madVR, as it works in 16-bit internally and has access to the original 4:2:0 data on the disc, whereas Lumagen products only process in 10-bit and have to be sent a 4:2:2, 4:4:4, or RGB signal as HDMI does not allow 4:2:0 transmission, so chroma upsampling must be performed inside the player. I have not done a direct comparison, as the last time I had the Radiance was shortly after its release, but madVR also offers some very good non-ringing scaling algorithms to choose from, which I suspect would rival or surpass the Radiance's scaling. (though I would like per-resolution options, as I prefer SoftCubic 80 for SD, and Jinc 3 AR for HD)
As the Lumagen products only process in 4:2:2 internally, it may be better to output an image scaled to 1080p anyway, to try and mitigate that loss of chroma resolution.
You may want to do color correction inside the Lumagen processor though, as it will have 10-bits of precision being output, whereas the output from madVR is only 8-bit right now, and yCMS could be better. (I only use it for gamma correction, not white balance or CMS)
I would have madVR set up to perform color space transformations though, which is achieved by telling madVR that "this display is already calibrated" to BT.709 - that way you only need to calibrate once as it will convert SD content to BT.709
