Will they hate me?

Whether they'll hate you depends on many factors:

- Is it an ensemble that specialises in contemporary music or players in a traditional orchestra? The first are usually open for doing lots of crazy stuff, the latter very rarely so.

- Is your piece included in a concert program with other pieces, or not? In the first case, retuning a string is problematic, since it won't keep its tune well after having been detuned. Generally, performers will take two different instruments for such a concert, but we're speaking of double-basses here, and even if they have a second one, they won't like having to carry along two of them.

- Most importantly: Did you speak with them before writing it? If you talk with performers, be polite and be ready to change your piece to something they can live with, they tend to be much more willing to work with you and do some crazy stuff than if you just write it, give them their parts and just expect them to play it. The more crazy stuff you want, the more important it is to actually get in contact with the performers directly.

As for tuning strings very low: In "nomos alpha" by Iannis Xenakis for solo cello, the C string is tuned a full octave down in the middle of the piece. It still gives a sound, but of course not your usual full cello sound, but it works nicely. The whole piece is extremely challenging though (it's probably one of the hardest pieces of solo cello literature) and it's generally only played by people who are very used to contemporary music. (It was written for Siegfried Palm.)

It is important in any case to keep in mind that after such a tuning action you can't expect the instrument staying in a steady tuning anymore. So either you use this until the end, or just live with a tuning that doesn't stay steady, or have them bring two instruments (but as I said, double-bassists certainly won't love that). Also keep in mind that retuning one string so extremely will not only affect this string, but the others as well, since the general tension on the instrument changes.

What I have done though, is specified that the double basses all have that low 'C' string, for range purposes. They're probably gonna hate me for that, if it ever sees the light of day.

Nah, they won't hate you for that. It's rather common. Not even to mention that these days it has become rather usual for bassists to play on five-string double-basses that also have a C string below the E string.

You can tune their C extension down to B, what's the point of going any lower?

Actually, a C-extension is only somewhat common in professional orchestras. Likewise, a five string bass would only be seen in professional situation as well. If you're writing a piece just to muck about with strings, placing these restrictions on it is going to guarantee that it most likely won't get played.