If you can use a solvent - just do it. I would try to pour a bit of the initiator into a beaker (not that difficult to weight) and dilute with the solvent. Solution should have much lower viscosity and would be probably easy to transfer into a flask. You may need some tricks to make sure you know the concentration, but it is all a matter of tracing masses of everything involved in the process.
Besides, if the initiator is that sticky even mixing it with other reagents can be tricky, so the solvent can make it easier to homogenize the solution and get the polymerization right in all the volume.