maybe you could use an invisble mini-terrain (btw are you taling about terrain or mesh?) that is recreated frequently, whenever the player reaches a new mini-cluster. The miniterrain has the same shape as the big terrain, only its only a few meters big, enough to cover the ground the player is walkng on. If this is a blitz terrain, it may be easy to implement it, simply determin the terrainX and Z of the position. Tho not sure if this will be slow, but when you do it not every loop, but only when the player reaches a new cluster (maybe something 3*3 based) then it could be useful.
Of course, this way you would only do collision checks with the mini terrain, that is alphaed to zero.
Of course, this way you would only do collision checks with the mini terrain, that is alphaed to zero.