Proportional fonts in tooltips?

The tooltips displayed by capi:display-tooltip on OS X are in a
fixed-width font, and noticeably differ from normal OS X tooltips
(e.g., those that appear if a mouse cursor hovers over, say, a toolbar
button).

Is there a way to get system-style tooltips?  Or at least to get
tooltips with proportional text?

Thanks in advance,
//JT