An executable that runs under Linux can be closed and proprietary. The fact that such executables are formed by linking against libraries leads to another issue that was solved by the LGPL (library GPL) that allowed the distribution and sale of closed-source products linked against (specifically) glibc. The combination of the LGPL (which allows linking against glibc) and the fact that you can run executables afterwards are what allow Oracle, for instance, to sell its product in closed-source form under Linux.
One could argue quite strongly that a package once mounted is much like an executable, making use of an API but not actually containing any GPL'd code. This would be true for uncustomized versions of the core and other packages you might work with. In other words, if the consumer can download OpenACS 4, install it, then install your package without your package modifying any of the underlying OpenACS 4 code then I think you'd be OK.
However note that all this is pretty fuzzy stuff. Richard Stallman has his own private interpretations of various licenses and whether or not they're compatible with the GPL, as well as what activities are or are not legal under the GPL. However none of this has ever been tested in court in any serious way.
