The package is not intended to be used on the stable distribution as you are doing, that's why you are having a painly install experience, it's not a package/age problem.
First of all, I don't think it's a good idea to have a stable/etch-backport/testing system mix.
Some openacs dependencies, like tdom 0.8.3, need a version of the libc6 more recent than the etch version, so you'll have a testing system even if you want it or not.
Also, this configuration has the problem of having to increase the size of apt cache to contain all the packages of the three repositories. I bet that the TCLLIBPATH problem is related to that config too, I have never seen it on lenny installations.
Finally, there is no need to add the compatibility options on postgresql.conf, the installer sets this options directly on the openacs database.