To help answer an OTN question today regarding the use of the domain/lib directory on WLS 10.3.3, I went to use the CAT utility (as I've become accustomed) and then realized we'd only added it from the WLS 10.3.4 onwards.
As far as I'm aware, we don't make use of any newer APIs in WLS 10.3.4 within CAT, so I simply copied out the wls-cat.war file from a WLS 10.3.4 installation and deployed it to the WLS 10.3.3 domain I was using for the test.
The WLS CAT application deployed successfully and then seemed to work normally, letting me locate exactly where class I was using was being loaded from on WLS 10.3.3.
Of course this is totally unsupported, but if you're in need of WLS CAT on an earlier 10.3.x release and of the adventurous spirit, give it a shot. You can always undeploy it if it doesn't work for you.