I have not received any notes from people saying that they have used the 6100 DOS card in their Quadra 800, but I did receive the following note from Bob Branick (robert.branick@bglobal.com) regarding using the original Houdini card in a Quadra 800. There aren't a lot of technical differences between the two cards, so I can assume from this information that the 6100 DOS card will work in a Quadra 800. Your mileage may vary, don't sue me if it doesn't work...
I was just reading your page on DOS cards transplanted into non-standard Mac boxes, and I noticed you hadn't heard from anyone with a Quadra 800.
Well, I'm using an original Houdini card (486sx-25) in my Quadra 800. I know this isn't the same model card as what you're referring to, but it at least indicates that the transplant is possible.
There isn't even any hacking necessary, at least with this model of Houdini. The only non-slick looking aspect to the operation is the way the video cables interact.
On the intended box, which was originally a 610, I think, you could mount a connector to the back of the case, occupying one of the punch-out "windows". The card attached to this connector from the inside, while the cable to your monitor attached to this connector from the outside. At least that's the way I think it was supposed to work...
Anyway, on the 800, there's too much distance from the card to the back of the case, so you can't do the neat mounting. You have to remove one of your "windows" in the back of the case, then run the external cable inside the box, where it hooks into the cable that's attached to the Houdini.
So, basically, you've got an open window into your case, which means that the hair-balls and small rodents that normally would get sucked into your floppy drive probably get sucked in through the window instead. I don't think it's fouled up the cooling; my machine has been running in this configuration for a couple of years, since Houdini I came out.