Werewolf stat changes should be implemented as damage/fortifications
When transforming into a werewolf Morrowind.exe looks at the player's current (potentially damaged) stats and then damages/fortifies them to the value specified by the various GMSTs. One side effect of this is that a player with more than 150 strength will see their strength in red while transformed. It also means players can restore their stats up to their non-werewolf values.
When transforming back the same happens in reverse. So a player with 50 strength, who used modstrength 1
while transformed, will have a damaged 50 strength after turning back.
As a sidenote, OpenMW currently lowers certain stats to 1 instead of the 0 they should be damaged to.