Dear Mark,
If you run mrregister without specifying the -type, it will default to -type affine_nonlinear, which does not include a rigid phase. As a result, all -rigid_* options do not come into play.
For the specific call (with implicit -type affine_nonlinear) you could add -affine_init_translation none and it would no longer perform this step as expected.
Should you want to include a rigid phase (e.g. with explicit -type rigid_affine_nonlinear) you should find that -rigid_init_translation none indeed skips the translation initialization for the rigid phase.
N.B.: I see that you intend to register multiple MRI modalities with mrregister. Know that mrregister currently only supports a mean-squared error metric to drive the registration. The implication is that mrregister is only appropriate if the images have the same contrast and intensity range (hence its only main deployment being in population_template). For the time being, you are probably much better off with a tool like ANTs for this use case. You can find tips on how to integrate this with your MRtrix pipeline on Registration: using transformations generated from other packages