Are you sure? I can't find any mention of ST_GEOMETRY in the Oracle documentation (Oracle® Spatial User's Guide and Reference:
http://download.oracle.com/docs/cd/B19306_01/appdev.102/b14255/toc.htm)
I know that there is an OGC-compatability package (also undocumented), which _is_ supported in HS (controlled by the the OGC_STRICT setting (see:
http://www.hibernatespatial.org/hibernate-spatial-oracle/usage.html)). It is my understanding that this is not exactly the same as what is provided in 11g under the name "SQL Multimedia Type Support".
This is also true, at least, from the Oracle 10g 10.2.0.2 version and up.