I haven't had the chance and may try this later. In any case, why would you want to use a file with a layer for creating a profile?
As far as I know, the idea is to have a completely unmodified raw, except for white balance
Well creating a tiff is never ever the same as "having a completely unmodified raw".
Just kidding.
I'm doing some more steps after importing the shot of the target:
base: "no color correction" profile, "linear scientific" curve, no shaperning, no noise reduction, no chromatic abberration, default adjustments for the rest
1. add a LCC to correct light falloff and color cast.
2. Shoot another picture with a white plane in front of the target. With this shot I create a layer to equalize the brightness for the whole area of the target (set the dark and white point on the levels tool as close as possible --> adjust with the brush on the layer). Of course I did everything before to get an "as perfect as possible" lumination of the target and the layer has only a slight brightness change (<0.1) with a very subtle mask. This layer is copied to the target-shot
3. adjust the brightness of the target shot to get the right lumination at 65% (a grey card with sRGB:165). This adjustment is also below 0.1. Otherwise I reshoot it.
4. Crop the image to the frame of the target.
Imo this give me better results than using the flatfield correction (for 1 and 2) in Lumariver and customized tone curves have the same brightness base (thanks to 3). As all this is possible with C1 and thanks to tethered shooting it's also not a big thing to get a good shot/check quickly, why not using these adjustments?