You can give task to Part creator to select the parent part. Create one IBA for WTPart let's say ParentPart write picker for that attribute/IBA and use that attribute/IBA as prefix in OIR for Number.
<AttrValue id="number" algorithm="com.ptc.windchill.enterprise.revisionControlled.server.impl.NumberGenerator">
<Attr id="ParentPart"/>
<Arg>-</Arg>
<Arg>{GEN:wt.enterprise.SequenceGenerator:WTPARTID_seq:5:0}</Arg>
</AttrValue>
Hope this helps !!!
Thanks,
Shreyas