A few points about the use of digital fonts in letterpress printing:
-=- OpenType does not support Multiple Master Technology.
-=- A very few MM fonts have an optical axis, which is especially
useful when printing items that include type of various sizes.
-=- The weight axis of MM fonts can be varied to compensate for ink
gain; lighter weights are used.
-=- Most people, I suspect, would find using only MM fonts a severe limitation.
-=- The digitizations of many of the classic Monotype faces are light
to begin with, as if they had been created from the face of the metal
type itself or from the original drawings, and not from inked proofs.
These can be successfully used in letterpress printing.
I have no experience in altering fonts in Fontographer or FontLab so
they can be used to print letterpress. I'd love to hear details from
someone who has.