Where it says:

} else if (!L.isString(v) && !L.isNumber(v)) {

there is no reason for the replacement value not to be null, true or false, they can all represented as strings. What it should check for is undefined:

} else if (L.isUndefined(v)) {

Also, it would be good to provide some sort of escape for including curly braces. As an extension of what I currently do:

Y.substitute(" {LBRACE} ", {LBRACE:'{',RBRACE:'}'});

those two identifiers could be built-in adding right before the first for(;;) loop:

o = Y.merge({LBRACE:'{',RBRACE:'}'}, o);