Is no possible in 6.3.5 code or prior, anything over 7.x code you can trace route from within the pix/ASA applience.,however, you can trace route from behind the PIX to any interface direction as long you allow it via acl, here is how to accomplish that.
http://www.cisco.com/en/US/products/hw/vpndevc/ps2030/products_tech_note09186a0080094e8a.shtml
As to why no traceroute from PIX code 6.x thats the way it was coded.
HTH
Jorge
Jorge Rodriguez