<texcode>
\starttext
\setuppapersize[A5][A5]
\setuppagenumbering[alternative=doublesided]
\starttext
\startpagemakeup[align=center,doublesided=no,%
Reference: http://www.ntg.nl/pipermail/ntg-context/2011/063030.html
Or, another solution (which can be tuned as at will), which relies on a
new feature in MkIV: there is also a layout with the same name as the
makeup environment and you can control the margins of your new title