It should not be extremely difficult to generate SGRS/SISR grammars as an alternative to Nuance grammars. SISR tags can handle about everything that Nuance semantics can handle.
The only issue I can foresee is that in a Nuance grammar, each RHS category can be referred to by a variable, as in:
REQUEST
(i want a SIZE:v_0 PIZZA:v_1 with TOPPING:v_2)
{return( (request $v_0 $v_1 $v_2) )}
In SISR only the latest category can be referred to, which means that semantics ("tags" in SISR) has to be interleaved:
$REQUEST = {out="request";} i want a $SIZE {out+=rules.latest();} $PIZZA {out+=rules.latest();} with $TOPPING {out+=rules.latest();};
More information can be found here:
SGRS definition: http://www.w3.org/TR/speech-grammar/
SISR definition: http://www.w3.org/TR/semantic-interpretation/
/Peter