Here's how to create a dotted/dashed barline between the staves while keeping solid barlines in the staves:
Create two overlapping groups - one with normal barlines on staves only, and one with dashed barlines between staves. Make sure that Use Alternate Group Barline is checked for each of them.
For the group with dashes between the staves, you can also displace the group name position a bit and put the name in the Hidden text style for easier reference while editing the score.