Figured it out. Instead of choosing 'Title Only' as slide type, choose 'Blank' and insert your own custom text box
import mlreportgen.ppt.*
slides = Presentation('test_temp');
add(slides,'Title Slide');
contents = find(slides,'Title');
replace(contents(1),'Test Name');
slide = add(slides,'Blank');
tb = TextBox();
tb.X = '0.44in';
tb.Y = '0in';
tb.Width = '12.16in';
tb.Height = '0.9in';
stitle = 'title1';
slide_title = Paragraph(stitle);
slide_title.FontSize = '24';
slide_title.Bold = true;
slide_title.FontColor = 'blue';
add(tb,slide_title);
add(slide,tb);