What about customTextElements ?
Finally I had to come back on this one and I've got it working :
In client side (javascript) :
printTemplate.format = MapConfig.printService.format;
printTemplate.layout = PrWidget.mapTemplatesFilteringSelect.get('displayedValue');
printTemplate.preserveScale = false;
printTemplate.layoutOptions = {
titleText:PrWidget.titleTextBox.get("value"),
customTextElements:[
{Entite:PrWidget.entiteTextBox.get("value")},
{Rue:PrWidget.rueTextBox.get("value")},
{CodePostal:PrWidget.codepostalTextBox.get("value")},
{Ville:PrWidget.villeTextBox.get("value")},
{Contact:PrWidget.contactTextBox.get("value")},
{Mail:PrWidget.mailTextBox.get("value")},
{Telephone:PrWidget.telephoneTextBox.get("value")},
{Fax:PrWidget.faxTextBox.get("value")}
]
};
It will automatically search for text elements in the mxd template base on textElement elementName that matches the keys (if I've got a textElement which elementName in "Entite" in my mxd, then it will update it with the value I defined).
This prevent to add "extraParameters" just for updating text in the template.