Can I request a cast for a suspected scaphoid fracture even if it doesn't show on xrays?

Yes. Yes...But repeating the xrays at two weeks post injury to confirm or deny the presence of a scaphoid fracture is still recommended. We don't want to needlessly immobilize a wrist -.
Splint not Cast. Most physicians would likely apply a splint rather than a cast for a suspected fracture. Repeat xrays would likely be obtained a week later to confirm or exclude the possibility of fracture.
Yes. Scaphoid fractures are sometimes difficult to visualize on x-rays, especially initially after an injury. If you have tenderness at the base of your thumb near the wrist, it is reasonable to splint or cast the wrist until follow-up x-rays confirm the absence of any fracture. Occasionally, an MRI study can be useful to rule out a scaphoid fracture definitively.