Remember that ACBC can be made to look quite a bit like just standard CBC. You can skip the BYO section. You can skip the consideration phase section. You can jump straight to the CBC-looking choices among triples (the tournament section). But, ACBC doesn't allow you to include a "None" option in the CBC-looking choice tournament section.
The big hangup with trying to use our CBC with constructed lists to do this is that the number of levels per attribute needs to be constant across people within the same CBC exercise. And, of course there is the messiness and manual effort to manually code your conjoint questions properly in the file for utility computation if you do constructed lists. If you start manipulating CBC designs by using constructed lists or (as you say you're thinking of doing) by doing fancy things on the fly to substitute levels that are relevant for the respondent for levels that are not, you need to have your strategy ready for all the manual data processing you'd need to do to prepare the data in the proper way to estimate utilities.
If somebody held a gun to my head and told me I had to do it in Sawtooth Software's CBC and I needed to create highly efficient designs on the fly that could accommodate between 2 to 7 levels of an attribute depending on the respondent's answers to previous questions, then I'd create six CBC exercises within the questionnaire. One of the exercises would have 2 levels of region; another with 3 levels of region, etc. Then, I'd use constructed lists and skip patterns to show just one of the CBC exercises to each respondent and to pass into the attribute the levels of region to display to the respondent. Then, I'd have the messy job of data processing to combine all the CBC data into one .CSV file and to recode the attribute levels for region so that level 1 meant level 1 across all respondents, etc. Furthermore, I'd want to add additional synthetic tasks to the data set for each respondent that communicated that dropped levels were not chosen versus included levels. That way utilities for dropped regions are strongly influenced to have lower utility than regions where the respondent has worked. After manipulating the .CSV data file containing the CBC data, I'd submit the data file to our standalone CBC/HB system for individual-level utility estimation.
It all is pretty messy and I would very much want to avoid all that effort and the data processing involved. It would be many hours of effort, for sure.