larfe,
It's semantics that's your problem. You can't go "past the point of maximum density." You can increase Dmax, however. Selenium toning (on some papers) will increase Dmax in the early stages, but as soon as a marked shift toward red-brown happens, the Dmax is being decreased. Many of us use light toning in selenium for precisely the slight increase in density that it gives. Density in other areas of the prints will increase as well. I've ruined more than one print because the toning dampened luminous shadows more than I liked.
Best,
Doremus