I expect it will work, when you restore simply choose maximum size for the VHD. WHS will restore to a disk (partition) that's identical or larger in size then the original disk(partition). One thing you should definitely do is test "bare metal restore" of your virtualized SBS to another vhd after you have finished the setup.
An alternative for backing up live vhd's is described
overhereEDIT: Please note this not only unsupported but also a violation of the EULA