The following example works with older versions of ConTeXt, such as the 
version from 20150325 in my repository. It fails with TL15 and later, 
including the current standalone beta.

    % macros=mkvi
    \setwidthof {99}\to\IDWidth\relax
    \def\STWidth{\dimexpr\textwidth-\IDWidth+1cm}
    \definelist[OList]
    \setuplist
    [OList][criterium=all,alternative=c,pagecommand=\gobbleoneargument]

    \starttexdefinition OLister #ID#String
    {\writetolist[OList]{}{\hbox{\simplealignedbox{\IDWidth}{left}{#ID\quad}
    {\doboundtext{#String}{\STWidth}{…}}}}}
    \stoptexdefinition

    \starttext
    \OLister1{Some list testing text xxx xxx xxx xxx xxx xxx xxx xxx xxx
    xxx xxx xxx xxx xxx xxx xxx xxx xxx xxx}
    \OLister2{Yet another list testing text xxx xxx xxx xxx xxx xxx xxx
    xxx xxx xxx xxx xxx xxx xxx xxx xxx xxx}
    \OLister3{A list testing text xxx xxx xxx xxx xxx xxx xxx xxx xxx
    xxx xxx xxx xxx xxx xxx xxx xxx xxx xxx xxx}
    \OLister4{Different list testing text xxx xxx xxx xxx xxx xxx xxx
    xxx xxx xxx xxx xxx xxx xxx xxx xxx xxx xxx}
    \subject{List half width}
    \startcolumns[n=2]
    \placelist[OList]
    \stopcolumns
    \stoptext

With the 20150325 ConTeXt, there are .tuc file entries like:

        ["second"]="\\hbox {\\simplealignedbox {\\IDWidth
    }{left}{2\\quad }{\\doboundtext {Yet another list testing text xxx
    xxx xxx xxx xxx xxx xxx xxx xxx xxx xxx xxx xxx xxx xxx xxx xxx
    xxx}{\\STWidth }{…}}}",

but with more recent versions, the corresponding entry is:

        ["second"]="\\hbox {\\simplealignedbox {12.6pt}{left}{2\\quad
    }{\\hbox {\\setbox \\scratchbox \\hbox {Yet another list testing
    text xxx xxx xxx xxx xxx xxx xxx xxx xxx xxx xxx xxx xxx xxx xxx xxx
    xxx xxx}\\scratchdimen \\dimexpr \\textwidth -12.6pt+1cm\\relax
    \\box \\scratchbox }}}",

-- 
Rik