Hi @tombishop
The only way that I can think, without changing the parser, is to send the separate variables in the serverside and then create the formula there (as I said also for polynomial regression):
For example instead of trying to pass the argument formula=“bmi.26 ~ noise.56 + female + (1|id)” you can pass three different arguments, e.g. formula.for.fixed.effects=“bmi.26 ~ noise.56 + female”, random.effects.before.the.pipe=“1”, random.effects.after.the.pipe=“id” (I know that these names for arguments are not good!).
Then, you can use R functions in the sevrerside to put the three arguments together and form the mixed model formula.