There have not been any additions to the Save HTML() function in JMP 15. However, you can simply read the file back in and change the title to whatever you want, and then save it back again.......as you suggested
Names Default To Here( 1 );
dt = Open( "$SAMPLE_DATA/Big Class.jmp" );
biv = dt << Bivariate( y( :weight ), x( :height ) );
rbiv = biv << report;
// Save the display
rbiv << Save HTML( "$TEMP/myversion2.html" );
// Read it back into JMP as a text string
myHTML = load text file("$TEMP/myversion2.HTML");
// Modify the "JMP Output", to whatever you want
myHTML = substr(myHTML, 1, contains(myHTML,"JMP Output")-1) || "This is the Title" ||
substr(myHTML, contains(myHTML,"JMP Output")+10);
// Save the file back to the original location
save text file("$TEMP/myversion.HTML", myHTML);
// Open the HTML file as validation
open("$TEMP/myversion.HTML");
Jim