Merge vertex group for Mirror modifier

I've often run into the situation where I have a mirror modifier and would like to merge some vertices along the mirror axis but not all of them. In these cases I always and end up having to split my mesh so one version can use the modifier with merge and the other without.

But it would be so much easier and make more sense if the modifier would support a vertex group, just like the Mask modifier, to select which vertices are merged or not so you could be selective about the merge process. Maybe even go beyond that and use the weight in that group as multiplier for the merge distance, might be of use for someone somewhen and with the default weight of 1.0 it's be just the same as on/off.