Well let's look at the reasons given - so far - as to why my suggestion is so terrible.

That posters would respond in PMs at length and tie the server up.
If this is a real concern - it (or the solution) is not a problem confined to my suggestion.

That posters may not be contactable.
Well the answer is to leave the post alone - as this 'indexing' is not a matter of life or death - just a matter of showing mutual respect.

The time and effort of the volunteers involved in asking would increase if they had to ask for prior permission.
This is one factor but if a job is worth doing - it is worth making the time and finding the effort to do it well. No one is forcing our volunteers to volunteer and if they find the demands too taxing - they can un-volunteer.

It is is not really honest to bring into this discussion of the routine imposed 'indexing' - the other fine work our volunteers may be doing to protect us from "vile personal attacks, spam, pornography and such" .