http://en.wikipedia.org/wiki/Film_speed (it also talks about digital camera ISO measurements).
Measuring ISO film speed depends from method chosen and some assumptions made in the ISO method.
Measuring of digital cameras' ISO sensitivity is even more complicated and up to a certain point arbitrary.
Digital ISO sensitivity when using raw is even more arbitrary or if I get it right impossible, just like it would be impossible to define the ISO sensitivity of a film without specifying the development method. The film with a latent image does not have an ISO rating per se.
Theory aside, I don't subscribe the idea that incident light metering cannot be used for digital cameras and the other way round. At most, one can find the deviation of the two (e.g. in the exact same conditions your DSLR 200 ISO is maybe equivalent to your reflected light meter 160 ISO, a third of stop off) and work from there obtaining reasonable values from a practical point of view. Over-thinking the matter doesn't help.
The rationale of bringing a DSLR instead of a separate light meter to help exposure with a film camera is certainly questionable, but that's another matter.
Separate light meters, and incident light metering in particular, are quite in use as far as I know also with digital cameras, not just with film cameras. E.g. in portrait work incident metering is used also when working with digital cameras, if I have to believe to videos I see on TV.
Modern separate light meters can be programmed with the dynamic range of the camera or film in use so as to better help exploiting the dynamic range without burning highlights (a problem with slide film and especially a problem with digital cameras because of the "cliff effect" or "wall effect" of the highlights).
The same separate light meters are employed also in digital photography which means the the ISO value makes some sense with both technologies (small correction factor apart if the manufacturer was a bit "optimistic" regarding ISO sensitivity for marketing reasons).
The ISO value of a digital camera is infinitely less useful an information than the ISO value of a film, and the logic in exposing a digital frame can be completely different than exposing a film frame (especially negative) so it's not so much the meaning of the ISO term in the two different technologies (which at the end of all the theory is in practice more or less the same) but the general exposure logic which can be completely different when the subject brightness range is extended.
Conclusion: a DSLR can be used as a light meter with a film camera if the subject brightness range does not pose problems, small repeatable differences apart. I wouldn't use it with complex light as the matrix metering might try to salvage highlights applying some sort of strategy which wouldn't fit film photography*.
* I wouldn't use matrix metering in any case, but that's me.