I've added the acting WHC members as additional information to the Country- and Year-pages (except for Yugoslavia, still juggling how to technically handle a country that has split up in many pieces).

To see certain trends (I noticed that the USA has been a Member during no less than 20 of the 32 years!), I think I have to start a separate page including other trends like:
- countries with the most whs
- ...

First Trends

to solve the Yugoslavia problem, you could do what they do in FIFA. A designated successor state, in this case, probably Serbia, is chosen to hold the history of the previous country.

yup, just include the note that it was a member before under the name yugoslavia. it's actually a common practice, even in united nations: russia as the successor state of ussr is the most prominent example there.