the template handler will be like erb template (maybe inherent from
ERB handler), but the final substituted result will be passed to some
unix tools for generating the final result.

looking at the source of rails template handlers. i understand that
any template handler should inhernt from TemplateHandler and define
compile method that will build a string of ruby code to be handed over
to rails for evaluation, and finally send the final result of this
evaluation to the client.

MY problem is: the final result will be a generated file on the
filesystem, and i couldn't figure how to throw send_file in the mix. i
looked at prawnto, rjs, builder and fleximage plugins. but all do not
generate results on the filesystem (aka do not use send_file).

However my hunch is that you'll have to manually set the relevant
headers for X-Sendfile and render an empty string.  If you're not
using X-Sendfile maybe you'll need to duplicate the behaviour that we
have in streaming.rb.

Given that revisiting the template API is one of yehuda's goals for
3.0, it could also be a good time to start thinking about additions to
the API to support this kind of thing.

--
Cheers

Koz