I found some neat graphics of a Carroll Transitway station buried in 300 N. LaSalle's PD application. That was in 2005, so these are at least 4 years old. There appears to be a direct-ish connection to the Merchandise Mart L station, which is a big plus, and a landscaped decked plaza over Carroll that is shared by Helene Curtis Building, 300 N LaSalle and EnV, in which the CTA stairs and such would be located. Note: the drawings refer to "rails" and a "curbed busway" so I guess they are meant to be noncommittal.
The drawings do answer some questions I had about how loading-dock and parking operations would work around the transit line. Basically, there would be two grade crossings of the line, at Lower LaSalle and at a point just south of EnV. I assume there would be appropriate signaling at these crossings, depending on the technology used (bus or light rail). 300 N LaSalle's parking entrance and loading dock entrance are along the east side under LaSalle, so they don't dump traffic into Carroll. EnV itself includes a roadway on the lower level connecting Kinzie and Carroll leading to the second grade crossing. The platform is set between the two crossings.
Plaza Level
Track Level