Some vendors seemingly offer to set the filename of a pre-connected unit via an environment variable. I'm not sure we need this, but some other compilers seem to have it and it is seemingly used: http://groups.google.com/group/comp.lang.fortran/browse_thread/thread/16d2c1d82a05edb4
I would strongly discourage adding this feature. A long time ago, either Paul Brook or Steven Bosscher and I exchanged email about the use of environmental variables to control various aspects of gfortran. In the end, I recall that we agreed a proliferation of environmental variables could lead to a maintenance nightmare. But, as usual if someone implements the feature, I won't stand in the way.
Tobias says "I'm not sure we need this", and Steve rightly points out that this was ruled out in the past. So, closing for now. (If the need arises, we should first discuss it on the list, IMHO.)