The topic was raised couple time, but yes, we are using an in between mod here based on a custom mod_rewrite from back when we where developing on phpBB2.
The idea was to perform some "research" at first and then to demonstrate how far could URL rewriting go. The mod never was released and actually stayed at the unreleasable state (stable though) because it implies too many changes and is not this much worth it.
The mixed mod like url where chosen for the same reason as ever, we though that support topic title would be hard to moderate for the advanced mod, with the experience we've had now, we may change our choice while converting, but nothing is decided. At list, this is a good proof that we do not give advices for other we do not follow : topic title injection may not be the working for all solution, especially now that phpBB3 support virtual folder as on phpBB SEO
We are now in the situation where our SEO premod for phpBB3
is more optimized than phpBB SEO itself actually, conversion will do some good