You can do it now, as of version 0.7 there is an additional option called 'maxNativeZoom' after which it will interpolate tiles.
Here's a link to the latest documentation for maxNativeZoom
Here's the info:
minZoom Number 0 Minimum zoom number.
maxZoom Number 18 Maximum zoom number.
maxNativeZoom Number null Maximum zoom number the tiles source has available. If it is specified, the tiles on all zoom levels higher than maxNativeZoom will be loaded from maxZoom level and auto-scaled.
A code example that I use to achieve it is here:
var aerial = new L.tileLayer(serverURL + "/arcgis/rest/services/Basemaps/Orthophoto/ImageServer/tile/{z}/{y}/{x}", {
attribution: attribution,
maxZoom: 20,
maxNativeZoom: 18
});