Looks like this may be the only solution for now.
Eventually if I got a new dedicated amp with a built in x-over (like my Crowm XLS 1500's) I can run the zone to it and split the sub and speakers, but till then it looks like this will be my workaround.
Sigh...if only there was a zone 2 sub pre out! Longing a bit here but oh well.
What about using the high/speaker level inputs on the sub instead if the line level inputs? If your sub has them, just feed the speaker wires to the subs high/speaker level inputs, and then connect the speakers to the subs speakers output.
For example, here is the back of a HSU sub showing all the inputs and outputs.