Is it safe to print a rotor insert for a centrifuge using a makerbot?
It's difficult to determine if the buckets are fully enclosed, but I suspect that they are. The enclosure into which the inserts are placed will provide some structural support. 

3D printed objects have relatively low torsion strength, but a reasonable compression strength, especially with high infill levels. One could consider that the item placed into the insert will transmit force to the bucket, but likely not compress to the point of destruction.

Allowing also that the forces on a swinging bucket centrifuge are "downward" or when in use, "outward," the primary location of force will be the bottom of the insert you create. It appears that the main function of the insert is to keep the tubes separated, rather than to keep them supported.

Create your inserts, but leave an open bottom. Attach a suitably strong panel to the bottom of your created model with epoxy.

Another thing to consider in this project is the mode of failure. What can go wrong? The tubes in the insert should not explode, but if they do, is it merely cleaning up that follows? I suppose one aspect of the inserts is to keep the tube walls in column, to keep all forces linear to the tube cylinder, rather than out of line. It's easy to keep pennies in a stack under pressure, but don't push on them edgewise.

You can and should perform a test on a 3D printed insert. Make one that is relatively flimsy, say a six-tube unit and spin it without tubes in place. Test it prior to the spin for tube fit, then test it after the spin. If there is deformation, you would not be able to insert the tubes after the spin.