Ok, i have tested the modified neck adding volume pot and it works well. There are some taper issues that mean that most of the adjustment is in the second half of the volume pots travel but that is fairly common for volume controls anyway! You can buy a version of this from rothwell that has a 'custom made Reverse Log Taper Potentiometer' that should function a little better in that respect.
http://www.rothwellaudioproducts.co.uk/html/neck_adder.htmlhere is stock strat wiring, i havnt drawn all the earth wires to keep it clear,

This is wired so the middle tone pot affects the neck pickup and the end tone pot affects the mid pickup. the tone pot for the mid pickup shares a capacitor with the neck pickup tone.
the first job is to remove two wires to disconnect the mid pickups tone pot:

This tone pot can then be removed and modified like i described above.
the tone pot is then re-installed and two new wires are put in. i have done these in green for clarity:

notice one goes to the neck pickups tag on the pickup switch and one goes to the volume control.
This is the basic mod completed. You now have an extra control that allows you normal strat operation when turned down but allows you to add the neck pickup to other selections when you start turning it up.
I just had a look at the rothwell neck adder fitting instructions that are online and this is as far as they go but hopefully the instructions that come with it are slightly more indepth
the problem at this stage is that you have a tone pot that only affects the neck pickup so i want to convert it to a master tone. There are two ways to do this. the first way would be to disconnect the tone control from the switch and re-attach it to the master volume pot - but we already have two wires there so its getting crowded. the other way is to connect a couple of tags on the switch like this:

if you dont want the bridge pickup to have tone then just connect the two tags near the end of the switch rather than all 3! I would do this extra stage with the rothwell neck adder as well
Sorted!!!
You now have master volume, master tone, 5-way switch that operates normally and a neck volume to add neck pickup if you want it but is bypassed by turning it down