To either heighten or smooth out the effect, you could use ligature support. Perhaps for double-letters (like the tt in letter) only change one of the pair.
Heck, while you are messing around with ligatures you could be really evil and use them to introduce typos, or unwanted corrections (color→colour)…
dspillett · · focus · HN ↗
Heck, while you are messing around with ligatures you could be really evil and use them to introduce typos, or unwanted corrections (color→colour)…