Is a d-dimer reliable enough to exclude a clot for someone who has polycythemia vera?

No: It depends where you set the cutoff / decision level. If there's reasonable suspicions of a thrombus, use imaging even if D-dimer is high-normal or "normal".

Not always: D-dimer should be used if the clinical suspicion for clot is relatively low, when a negative test is quite reliable in excluding clots. However, because of higher baseline risk in PV there is often a higher clinical suspicion, and negative d-dimer may be insufficient to exclude clot. Discuss more workup, like ultrasound, if typical symptoms of clot are present.