seems to be the core problem. When I changed that to what is shown
above, I get reasonable values for my test data.
Ignore the fact that my original post had +x_0=3500000 and +lon_0=9.0
instead of +x_0=4500000 and +lon_0=12. I had been trying many
combinations and only posted the most recent. It was only the change to
+towgs94 that made any difference.
Here is another mystery:
I updated my epsg file to my preferred definition. Then, when I use
+init=epgs:13468 in the C API, I get locations that are 10 meters or so
away from when I specify the same values 'by hand' in the C API. My
entry in the file is: