Here is a link to a recent paper in Crystal Growth & Design:
http://pubs.acs.org/doi/abs/10.1021/cg301314w
This details co-crystallisation of PZQ with a variety of organic di-acids, and single-crystal x-ray analysis of their structures.  Such crystals are of potential use in formulation as they often alter the solubility and dissolution properties of the API.  NB co-crystals are formed despite the inability to create a true salt of PZQ. 
Interestingly, the crystallographic study showed that there are two main rotational conformers for PZQ, which differ in the configuration of the C=O groups in the piperazinone-cyclohexylcarbonyl segment.  In rac-PZQ crystals (ie not a co-crystal) a syn-conformation is seen, whereas for all but one co-crystals it is anti-.  Reference is also made to (R)- and (S)- PZQ crystals as being hemi-hydrates, and also showing anti- configuration.
The chiral di-acids named in the abstract were all simple homologs of succinic acid or unsaturated equivalents.  No mention was made of any chiral acids being trialled.  It might in theory be possible to achieve a resolution by co-crystallisation despite the inability to form a salt !
 
regards, Nick