Local time signature

I've got an SATB score with piano accompaniment. In a section in 4/4, I want to have the top three parts (SAT) in a mixture of 3/4 and 2/4 while the bass continues in 4/4. I've tried every which way to do this using CTRL and dragging the desired signature from the palette, but I still have two problems.

1. I try creating a local time signature, but a popup tells me it can't paste one because the bar isn't empty. I've tried everything I can to empty it: deleting absolutely everything in the bar, inserting an additional 10 bars so as to work with that, saving the new score as an experimental version and working with that. Nothing doing - the popup still pops up. The only way is to create a new score. That works. Trouble is, problem 2 crops up.

2. What happens when it does succeed is bar=bar - i.e. a bar of 3/4 is the same length as one of 4/4, meaning that the former sounds like a triplet compared to the latter because the crotchets are one third again as long. What I want is crotchet=crotchet - i.e. the crotchets proceed at the same time as each other across all four parts, just grouped differently between SAT (a mixture of 3/4 and 2/4) on the one hand and the bass (4/4 throughout) on the other.

Any ideas?