Assuming the camera system gives an edge-on view of a VA cloud ahead of the aircraft, there's an immediate problem with ranging (ie. distance to the near and far edges of the cloud). Even if it's possible to determine these points and therefore the total depth of the field being observed, it still seems unlikely that an accurate ash-density number could be calculated per cubic metre. Once the near edge of the cloud is close to the aircraft (and therefore flying through it or not comes into question), it seems very unlikely that an InfraRed imaging system would give accurate (or indeed, any) data that could be used to calculate actual exposure.
This thought already led me to comment that a vertical shot through a cloud will be a much richer data-source. VA clouds tend to cover large areas in relatively thin layer(s), which will be much more easily measured and analysed from above or below (although such layers are much easier to see edge-on, as a dark line, using the Mk 1 Eyeball, this is not the same thing at all).
For these reasons, I cannot see how the proposed passive IR system could provide consistent or accurate data about VA actually flown through.