I have added a Calculated Value control to my form (I hide it using custom css). I have set name to IsBuyingDept and the formula to fn-IsMemberOfGroup(Buying Department). Note that you can't use a reference to add this formula, you have to type it in, but it works and will set the value to true if the user is a member of the Buying Department group, and false if he is not.
Then I've added the following rule to my panel to hide it if the user is not a member of the buying department:
And now the value in the supplier drop down (that is inside the panel so hidden to non members) is preserved if a non member saves the form.
A bit of a long way round but it works :-)