I have this feeling that no answer is going to be acceptable.
Try creating an ordinary input cell containing
NumberForm[N[Pi, 12], DigitBlock -> 3, NumberSeparator -> "_"]
Evaluate that.
Then create a title cell with contents, but without the number you wish to display.
Scrape the output from the NumberForm evaluation.
Paste that into the appropriate position in the title cell.
If quotes appear around that paste then try to remove them.
When I try to directly evaluate in place a NumberForm expression within a title cell this does not seem to work.
When I enter long strings of digits into a text cell there is no default automatic reformatting of digit blocks.
