You could actually get the values from the font object using brightscript and then use these values in the getFont call.
So if you had your label like:
<Label
id="myLabel"
text="Awesome"
translation="[300,300]" >
<Font role = "font" uri = "pkg:/components/fonts/Helvetica.otf" size = "24" />
</Label>
You could get the values of this font by doing something like:
myLabel = m.top.findNode("myLabel")
myFont = myLabel.font
fontURI = myFont.uri
fontSize = myFont.size
You could then use fontURI and fontSize in your
roFontRegistry getFont calls.
Hope this helps.