I see only one way to make this without too many new custom parts + modular with the complete line of existing cases or as many as possible + for a price that people would still want to buy it
Here is my proposel.
Note: I do not know how the part is called that's used to attach all the panels. The things that clamp with the hole and on the panel there is the thing sticking out. I am going to call them
dinglies.

This is a Caselabs case looking from the side.
Orange is the case. On top it has dinglies which are normally used to attach the top cover.
Green is the new part that has to be made. Instead of mounting the cover on top of the case, we mount our swtx tray on top of the case. The swtx tray itself also has dinglies on top of it.
Red is our normal 85mm top cover. Because of the dinglies on the swtx tray, the allready existing cover can be attached.
Yellow is our swtx motherboard.
Here is a schematic from the top and side with a bit more detail.

The plates on top of the case normally meant for 2x 120.4 rads are removed. Thanks to the holes in the swtx tray plate, you can lead all hard disk and power cables down into the case. So the user can use the hard drive cages and psu mounts of the case.
This system allows CL to pretty easily make the swtx tray for the M8, M10, MH10, T10, TH10 and TX10. It would work for new customers but also for all existing customers.
The tray is either made from one whole part. This means 3 versions of the whole tray and cutouts and motherboard mounts should be made. M8/M10/T10 version + MH10/TH10 version + TX10 version
Or the tray would be made from 2 parts. 1 part is the base with the dinglies. 3 versions of this base have to be made but it's an easy structure. The 2nd part is the swtx tray with cutouts and motherboard mounts part which is a universal size that fits on all the 3 versions of the base plate mount.
Does this make sense?
EDIT: this design is assuming it's run headless and with watercooling. If a screen needs to be attached you can take the cover off and connect a screen while you are busy. If high profile air cooling blocks are needed, there will have to come an extended top larger than 85mm. Network connection isn't a problem because a utp connector can fit in there and be run via a watercooling hole in the case to the back.
Edited by StijnDP - 8/31/12 at 5:46am