yours is a jazz background and perhaps you are used to speaking of modulation even for phenomena of this type.
Not necessarily. Even the basis of tonal Jazz does not deviate much from the "classical" one. As far as I am concerned, there is a difference between a modulation and a tonicization, or however one wants to call it. Of course, I understand Modale's point of view; in fact, at the moment when one has to improvise, the secondary dominant is considered a dominant for all intents and purposes. But it is not always like that...
Anyway, why not use a VI of melodic Eb minor when modulating from Eb minor to Bb minor? Like Cø F7 Bb-... The transition to the Bb dominant would be less traumatic...
