\setupbodyfont[dejavu]

\startluacode
    fonts.handlers.otf.addfeature {
        name = "pairtest",
        type = "pair",
        data = {
            A = { B = { { -30, 50, -50, 0 } } },
            B = { C = { { -30, 0, -50, 0 } } },
        }
    }
\stopluacode

\starttext

\definecolor[tgray][s=.5,t=.5,a=1] \showfontkerns \showglyphs

\definefontfeature[pairtest] [mode=node,pairtest=yes]

\definedfont[file:dejavu-serif.ttf*pairtest @ 48pt]

\startTEXpage[offset=10pt,foregroundcolor=tgray]
    ABCDEF
\stopTEXpage

\stoptext