If a Span is necessary or preferred for your application, the following workaround might be usable.
Create a glyph such as a circle in addition to your span and place it at the same x- coordinate. Set the glyph’s alpha(s) to 0.000 so that it is invisible. Attach a HoverTool to the glyph’s renderer, with the hit-test mode set to vline so that whenever the mouse is at the same x-coordinate (i.e. vertical line), the hovertool activates.