I ended up using foreach method to manually filter the result of catalog_category.assignedProducts method like this:
$products = $client->call($session, 'catalog_category.assignedProducts',$categoryId);
if($brandId!=null && $brandId>0)
{
$result=array();
foreach ($products as $product) {
if($product['brand']==$brandId)
array_push($result,$product);
}
}
else{
$result=$products;
}