Not sure about US laws and regulations on this but can't you attempt to transfer your existing positions between FCMs?
I trade STIRs, the spread markets are quite thick, but some butterflies or double butterflies not really. Of course I can cross the spread but I'd rather avoid this.
And unwinding in some cases is not possible as I am in liquidation mode and unwinding a position on a legged double butterfly involves expanding my position while legging out...
My only hope now is to find a new FCM and transfer my account quickly enough. I have also asked my account rep to get an extension (I have unwound a position already).