This was fixed some weeks ago as part of bug #47503 (see http://hg.code.sf.net/p/octave/geometry/rev/5850349b8181 )

Yes, this is an elegant solution.  I would just suggest to use "internal" names like _pkg_add_geometry_ and __pkg_del_geometry.

I have thought about this, but where would such a function be placed?

When loading/unloading the geometry package, some variables defined by the users may be clobbered, like in the example below:

octave:1> pkg unload geometry
octave:2> ii = 0;
octave:3> pkg load geometry
octave:4> ii
error: 'ii' undefined near line 4 column 1

This happens because the scripts PKG_{ADD,DEL} use and clear a series of variables.  One possible solution for the problem is provided in the patch attached to this bug report. This patch is pretty minimal and just wraps the commands in both PKG_ADD and PKG_DEL inside functions with quite "unlikely" names _geometry_pkg{add,del}__, which are called afterwards. These functions are cleared at the end of the scripts.

This problem also affects other Octave-Forge packages.  I will try to track them down soon.