This would definitely work better than using their SR as turn resistance. Some outsiders have a really good SR and that would clearly go in favor of the outsider (though I myself dont have a problem with that..my players might, but I dont )
Yeppers- and remember any outsider (lower planes creature in 1e) with an AC better than -5, 11 or more HD, or 66%+ magic resistance was immune. (pg 76 in the 1e DMG)
To mimic that in 3e, just don't let clerics turn outsiders that have one of the following: more than 16 HD (since the malebranche in the MM2 has 16), an AC better than 28, or SR 24 or better.
I fudged the AC a bit (-5 should be AC 25) so a cleric using this system could still turn a glabrezu. In 1e they could turn all demons except the princes, lords, mariliths (AC too low, magic resistance too high) and balors (magic resistance too high). This lets them still do it that way (except they cant turn Nalfeshee...though you could bump the AC and SR by a point to fix this).
Also- why not just set an outsider's turn resistance equal to 1/2 its HD?? Might be easier that way.