Ah well, it makes sense, Ham uses the virtual table in the mod ; but MonsterMod is a bit special, it basically uses HLSDK as base but modified a bit to be a metamod plugin. So, you will need to hook TakeDamage in MonsterMod. You can probably use Orpheu, filling "library" with "monstermod", maybe removing "mod" and using the right indexes. This should be the same as HL1 (mod "valve"). Though it might be tricky and you may need to recompile MonsterMod the same way as the game.
You can also hook the CBasePlayer::TakeDamage with a signature in case you fail to deal with the virtual function. This should work if you don't forget to specify "monstermod".