If you do it in this (accepted) way you’re stuck with sharing gain… but then again how much difference would you need to have with the FOH engineer?
Let me back up – is this system always being used by the same group/engineer? If so, you’d assume that the gain settings are reasonably static from show to show – in that case the FOH engineer may be open to setting trims on surface at FOH too (once the gains have been set with a reasonable amount of headroom), which means you’re not going to get any sudden changes from them adjusting the head amp.
You’re right, if you don’t want to share gain you’re down to splitting your signal pre the iDR (i.e. into separate inputs), but I’d suggest you’d find it easier to have this conversation with your engineer than anyone on here 
iDR32 mixrack
Asus N61 Laptop (x2 wireless)
Sennheiser IEM (for PFL)
Dante card (soon!)
