Batch Rename

Was about to post an idea to renaming stack file and stumble upon your problem. If you had not found a better way, perhaps the following would be a fit for your use case.
1. I use face detect. Lightroom had gotten much better at this (for ones it already had in it catalog).  If I understand you already have that so may be be relevant. What I do is all people (ie face detect) have a prefix with a special character ie %john.smith (I always combine the names with dots which comes in useful in filter searches).

2. Once you have those keywords you can use the filter library to sieve out photos with the keyword ex %john.smith
3. Then do a crtl-A and from the meta-data panel add the name to (recommended) caption field ie John Smith.
4. Setup a filenaming preset with the caption in the renaming string.
5. All you need to do when you had added new photos and populated the captions is batch rename using the filename preset. 

Claus,

I can't think of a faster way to process the file name change than what you are already doing.  Maybe keyword by name first then you can easily sort in the metadata filter bar by name to ease the renaming process without using stacks.  Should be faster especially if you are keywording names anyway.

Is there a better method for segregating the images when taken such that they are imported to a different directory for each person?  Some cameras allow you to change folders - maybe creating a new folder each time you change subjects (as long as they are shot in succession) would segregate them on import if you have not elected to ignore camera folders.

Just a thought.  Didn't want you to think everyone was ignoring your question.

Jeff