have you made a mat box lit from the front ?
you would mount the underexposed film in like a frame
and look at it against a black background cross lit.
the film would be mounted whatever the sweet-spot distance
is to make you view it as a positive.
i have often thought of how to make my current film look like tintypes
and i haven't been able to come up with a solution other than what i just wrote
... unless of course you want to re-coat black photo paper with liquid emulsion
( or over exposed "black" film )
and then use ferri or some sort of bleach ( or ? ) to brighten what will become your whites ..
its a lot of work but who knows ...

if i come up with anything i'll let you know