Is it okay to practice calligraphy with sandbags tied to your hands?

Not desirable. Wrist strength in calligraphy is not entirely the size of strength, but more importantly the flexibility of the wrist. The muscles that bind sandbags will be over-strained, and the lines written will be stiff. Moreover, after unbinding, the hand will be light, the writing font will float, and there is no strength. The correct writing should be very soft and relaxed.