I don't know anything specifically about Custom Visuals in this context, however, the way this is normally done with standard visuals is to turn off the standard title and to display a measure whose value is dynamic text within a single item card placed above the report. You use a series of IF and HASONEVALUE statements in the measure definition to control the text output depending on the slicer filtering or cross-filtering that has been applied by the user.
As I understand it, custom visuals deliver cross filtering the same as standard visualisations.
If it absolutely has to be integrated from within your custom visualisation, is there anyway to include such a text measure in what is passed to the visualisation?
Hope that helps