Well to begin with there is no easy way out.
OPTION-1 : Use XCart feature in XPSExplorer to export all XID of objects that you wish to export. Then use the XCart file as an input to XPSExport in R12.0. This would only export objects which are listed within the Xcart file.
OPTION-2 : Use smobjexport to do a full policy store export in smdif format on the R12.0 Environment. In <R12.0 Install folder>/db there should be a smpolicy.smdif. Do a file compare of R12.0 smpolicy.smdif and R12.0 smobjexport file. Remove all matching entries (i.e. default objects - because smpolicy.smdif has only default objects). Similarly if you'd have federated objects, then ampolicy.smdif in R12.0.
OPTION-3 : Try to use R12.52 SP1 or R12.6. These version have much better & stricter logistics in terms of Object import e.g. R12.52 when doing a import finds a object with same name exists in policy store but with a different XID, it falls back to Name match instead of the old method of purely XID match.
OPTION-4 : Using SMPolicyReader to generate xcart selection.
Would let know if I can think of anything more.