Instead of radiusing the underside of the shelf remove the radius from the fretboard under the shelf,

The main challenge, really, is the player's preferences. He tunes down 1/2 step for vocal reasons. He plays primarily in first position. His grip on the left hand pushes the strings almost to the fretboard or possibly to the fretboard. He makes strings go sharp even when playing with a capo. And he uses a pick, hard, so the action needs to be fairly high to stop buzzing.

Dave I am sure you have already thought of this but I'll throw it up anyway. Could you cut a new saddle slot to allow a wider saddle that would enable proper saddle compensation? I realise you have to have enough room in front of the pegs but maybe it is a possibility.

Cheers Dave

Thank you for your responses so far. Still hoping for a milling idea.
I did not want to do anything invasive and irreversible. I am sure the owner would not be happy if I did.

Hesh... Thanks for being predictably helpful. I learned to cut nut slots (Charles Fox) exactly as you do them, and, although I don't do as many as you do, nor as fast, these are suitably low. And I agree that this eliminates most noticeable intonation issues. This was nowhere near enough for this case.
As for the type of guitar, a major USA made company, probably the most recognizable name in acoustic guitars in the US. It has a 3/32" saddle that is at least .1" short of where it should be for correct saddle intonation even after shaping it to push the string contact as far back as possible. This contributes to the need for compensating every string (compensation at the nut reduces the amount of compensation required at the saddle). The year of manufacture is not in the range of "misplaced bridges" it is just a typically (incorrectly) built guitar that is acceptable for 95% of the people but not for everyone.
The main challenge, really, is the player's preferences. He tunes down 1/2 step for vocal reasons. He plays primarily in first position. His grip on the left hand pushes the strings almost to the fretboard or possibly to the fretboard. He makes strings go sharp even when playing with a capo. And he uses a pick, hard, so the action needs to be fairly high to stop buzzing. Add all these requests and guitar limitations together, and nut compensation (I know you don't believe in it) is the appropriate solution.
A search on this forum would yield a post by Alan Carruth about a similar situation with a 12 string.

thanks again
Dave

Further to what John said, if the saddle is out of position by .10 of an inch and you intonate at the half scale position
and move the nut by that large amount the remaining frets, other than the twelfth assuming a standard guitar, are all now in the wrong position making tuning difficult at best. Since you are only concerned about the lower fret positions it seems you are
fudging things enough to satisfy the owner. So I needn't say any more.

As for milling the shelf nut, here's one approach. Use a carrier and router or laminate trimmer mounted in a table or temporary
setup to do the job. Take a piece of 3/4" stock say a foot long and route or sand the fretboard matching profile into the side a couple inches from the end. On top route or saw a rabbet for the nut blank to sit, the fretboard profile will now be under the
nut blank. A drop of CA to hold the nut. Use a top mount bearing flush cut router bit and make a temporary fence from a piece of scrap with a slot cut in center so as to cover the bit with a little clearance. Route the blank by incrementally moving the fence
exposing a little more of the bit with each pass. When the bearing reaches the bottom of the fretboard profile you're done. Some acetone will release the CA. Any ole smallish flush cut router bit probably could be used with a slightly different approach.
Nothing beats a drawing, I know. Hopefully you get the picture though.

I make my compensated nut using an end mill. I think if I was trying to do what you are I would mill a straight cut at the dept of slightly shallower than the final depth at the edges of the final nut shelf and use a scraper to put in the radius. Scraping and checking, repeat until it fits.

I have done it two ways. One with a router in a bishop cochran router base and a jig that allowed me to hold the nut and guides that allowed me to move the router parallel to the nut. That gave me the two axis of control I need to make a shelf. I used to use the same jig for my saddle slots.





Now I have a mini mill. That I use for a lot of stuff. It gives me three controlled axis to work with. More with special vises.



I run everything full speed. Bone mills, cuts and scrapes well.

I have use 1/8"-1/4" end mills depending on what I am doing. To make the shelf I would probably use a 1/4"