The problem here is that your Dashboard could contain multiple sheets, each one of which using a different data source. So <InvoiceDate> taken in a vacuum wouldn't help. Is it the <InvoiceDate> from DataSource1 driving sheet1? or maybe the <InvoiceDate> from DataSource3 which drives sheet3...you get my drift.
Anyway, long story short, the Dashboard title isn't going to perform the way you want it to in this regard (although it would be cool if it could).
Most people will work around this by creating another sheet which does nothing else but show:
I can't get this Title to work on a Dashboard, the invoice date is: <InvoiceDate>
...You'll change the font size / font name to make it look like it's a Dashboard Title, and then position this sucker at the top of your dashboard to act as a surrogate dashboard title. One extra step, but it works!
Great, i'll use this workaround!
I could imagine a world where you specify something like this.
Yup! That'd be a nice little feature, actually!