That's quite interesting, it certainly is difficult to work out extinction when there is not any "markers" to compare against. The great thing about working out extinction of star clusters is that you have a lot of stars within a cluster to create a benchmark. Calculating the extinction caused by the Milky Way is not as difficult because we at least have that patch of sky to work with. Extinction within another galaxy, we have the supernova and thats it
This is where dealing with Type Ia supernovae are fantastic, they are considered to be standard candles because they shine with the same brightness (changes with size but it is very well modelled).