[XeTeX] Smart quotes with \font?

[Kenneth Reid Beesley]

When using XeTeX and TeX-style \font commands (rather than fontspec),
how can I make ``foo'' (surrounded with two left single quotes on the  
left
and two neutral or right single quotes on the right) get set with  
smart double
quotes?

Or is there another way?  Obviously I could just reedit the source  
file and
substitute real Unicode smart quotes...

Thanks,

Ken