I’ve been looking for a way to turn on force duration by default. Is this possible?
No. You’re not missing anything. You have to turn it on manually when you need it.
As someone whose just started to get into Dorico, I have to admit that this thorn has really been a test of my will. What happens if I want to write a piece in 7/8 in which every bar is different?
Thanks for the help, dankreider!
You can define the beat structure for a time signature, if you enter something like [3+4]/8 or [2+3+2]/8 into the popover. (Note, 3+4/8 will display what you typed, but [3+4]/8 will display just 7/8 in the score). There are other options for defining alternating time signatures, etc - see the documentation.
If you really have different rhythmic divisions in every bar of a piece, I wonder how hard it would be to read.