I have no experience of this kind of labs but, as usual, I would like to give a suggestion
Even if the minilab doesn't have an explicit "zero filtering" position, it should have the possibility to filter all images with the same filtering, some kind of a "filter lock".
So you might photograph, in the first exposure of the film, a grey card in daylight, and tell the operator to lock filtering after the first image (meaning, all subsequent images to be filtered like the first).
That way, if you take some pictures with sunset light, having "orange greys", the machine will not convert them to neutral greys and your sunset will look like a sunset (and probably even too much so).
By the same token, if the machine can lock exposure, you can have the operator lock also exposure, besides filtering, on the first image (18% grey), and if you take a picture which is 4 EVs overexposed, you will get it actually 4 EV overexposed.
I don't see the usefulness of locking exposure with negatives, but I would understand the usefulness of locking filtering if the process is automated (not operator-assisted). That would be equivalent to shooting slides without filtering: very orange sunsets, blue casts on open shade. If you use instead CC filters, by locking filtering on the first exposure of a grey card in daylight you should end up with exactly the filtering you want even if the machine is totally automatic.
Fabrizio