colspan isn’t one of the available options with a quick cell. You might instead use $DSP->td(), whose parameters are: style, width, colspan, rowspan, valign, align. However, note that td() and table_qcell() are both deprecated:
// DEPRECATED DISPLAY FUNCTIONS
// ------------------------------------------------------------------------
// ------------------------------------------------------------------------
// At present we're still using these so they have to stay here.
// Once we've gone throgh the entire control panel and replaced these
// function calls with the new versions we can kill them.
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