using a spot meter as an incident meter!

[Sevo]

It seems to me that the disk will remove some of the light reaching the sensor, and thus should be accounted for in some way.

It better should remove some light, or the meter would be way off. Meters generally are calibrated for reflective reading off a about 15-20% reflective surface. So all that is needed is a corresponding transmission value for the dome or disk - which is trivial to achieve. All meters I am aware of need no separate adjustment for incident.

[Bill Burk]

Here's my opinion, and I'm starting to figure it out... It is a rough and casual opinion, not intended to be scientifically accurate or unimpeachable...

Incident reading can be done off the dome in the meter.

Aiming a reflected light meter at the palm or ball of hand... or gray card or any other "known" reflectance and then "placing" that reading "where it belongs" accomplishes the same thing.

You are evaluating the amount of light falling upon the scene and calculating exposure based on that.

But...

Aiming an averaging reflected light meter at a scene, or using a center-weighted meter inside a camera. This depends on the distribution of light in the scene and works on the law of averages to give you exposures that, on average, should be right. Black cat in coal mine or Adobe on white sands are going to be thrown off.

... And... Spotmetering and placing readings on Zones is yet another beast altogether.

[Stephen Benskin]

One other note. None of the other dome/spheres I have will work. They're all too dark. Only the "Lumisphere" from the Sekonic 398 will work as far as I know...

Do you happen to have the values for C for those meters?