I would not count on it either.
We've been to soft openings of a number of attractions and because of how they work, they are unpredictable.
They tend to open and close abruptly and, until the end soft openings (like those for AP holders or DVC members), they are not open at a specific time or for a specific time.
Except for those, they don't announce the opening to anyone - it's pretty much, if you are walking by and see people in line, it's open right now. But, no guarantee for how long or when they will close the line.
As people see it's open, they tend to post on social media and discussion boards, so people come running to get on before it closes. Because of delays in posting and/or people reading about it, guests often get there after the line has closed. That can cause somewhat of a mob scene (which they are trying to avoid by not announcing).
The soft opening is to test things, so the length of the opening can depend on what they are testing and may be short. Lines tend to not be very long waits because they only allow enough people in to do whatever they are testing and they close the line to new people before the wait gets long usually.
In the early days, there tend to be more technical tests and if something comes up, they stop the attraction to fix it. So, you could be in line and the line is stopped for an unknown period of time or evacuated. You could also be in the attraction when that happens.
Later tests tend to be more practice for CMs, so tend to have less stoppages. But, again, they have goals for the test which determine how long the test will be.
Because of the testing, they may not have a lot of flexibility available for doing
DAS, so might not allow it for that reason. Even after the technical tests are complete, there may be more stoppages.