I have a set of Canopus SLI cards and a spectra 2500.
First of all: The internal loopthrough is called Whichdoctor and it needs it's own drivers. As far as I can remember they are just available for Win9x and NT (<-not entirely sure about that).
There are two ways of using the Spectra together with an V1/V2 board: You can loop any signal coming from an 3d only accelerator through the spectra using the external connector. This is like the normal loop, but just the other way around (3D->2D->Screen, instead of 2D->3D->Screen).
I had that setup up and running smoothly using Win98SE, 2D quality was visibly improved on a TFT (no CRT to test). 3D was a little more blury than 2D, but still ok (in my opinion a little better than the ordinary solution).
The second way is the internal connector of the Spectra, which only fits with Canopus V1 and V2 boards. Sadly sometimes you do find a Spectra but without the cable neccessary. I only once tried to use that, but sadly there seems to be a mismatch with the motherboard or the driver, cause the system crashes when trying the internal connection using 3D acclereration
To be honest, I don't really have an idea how rare the cards and items are, I found one a long time ago for quite some money.
If you interested, I'm actually selling the cards
http://www.falconfly.de/cgi-bin/yabb2/YaBB.pl?num=1298208353