If you want native multi-select, you're going to have to use a multiple-select browse widget as opposed to the standard browse used by a smartDataBrowse as default. Not sure that this is available in v8 though. Time to upgrade!
Back in the day, when dinosaur-shaped single-select browse widgets roamed the landscape, I used to use a temp-table based on a real table, with an extra selectFlag logical field. This was the first column in the browse, and the column was fixed (so always visible). I'd use the row-display trigger to color selected rows in red. Thus, I "sort of" had a multi-select browse object.