Solution was simple. As I suspected .OnBeforeRequest("addSomeOtherID(grid)") did the trick.
function addGridSearchOption(grid, fieldName, fieldValue) {
if (grid[0].p.search == false) {
f = { groupOp: "OR", rules: [] };
f.rules.push({ field: fieldName, op: "cn", data: fieldValue });
grid[0].p.search = true;
$.extend(grid[0].p.postData, { filters: JSON.stringify(f) });
}
else {
tFilters = eval('(' + grid[0].p.postData.filters + ')');
var bAddRule = true;
var i = 0;
for (var tRule in tFilters.rules)
{
if (tFilters.rules[i].field == fieldName)
{
tFilters.rules[i].data = fieldValue;
bAddRule = false;
}
i++;
}
if (bAddRule) {
tFilters.rules.push({ field: fieldName, op: "cn", data: fieldValue });
}grid[0].p.postData.filters = JSON.stringify(tFilters);
}
}