variable substitution within a projection object

[Bruce Saunders]

I'd like to be able to pass in a variable to be in substituted in the
projection object but it doesn't seem to work as expected.

I'd like to do something like this:

  PROJECTION
    "proj=utm"
    "ellps=GRS80"
    "lon_0=%lon_0%"
  END

It seems like the variable isn't getting evaluated in this context.
I've verified that the value is getting passed correctly since it's
getting correctly replaced in the template.  I've tried replacing the
entire value with something like:

  PROJECTION
    "proj=utm"
    "ellps=GRS80"
    "%lon_0%"
  END

But that doesn't seem to work either.  Are variables not supported for
projection objects?  I'd appreciate any thought or suggestions people
may have.

-Bruce