access to inherited variables from within a template

This is where "Puppet isn't Ruby" bites hard. I have no idea what a fully-qualified Puppet variable might look like in ERb, but I suspect it wouldn't be pretty.

I think you've got a bigger problem than ERb syntax, though -- the idea of "one host [configuring] another one's config files" just scares the willies out of me, and I suggest you rethink your manifest design somewhat.

As a workaround, have you tried doing this in your template:

  var: <%= scope.lookupvar("someclass::somevar") %>

That worked for me for a very simple example.

Use the workaround mentioned by lutter.