Seems I was using it correctly but was accidentally overwriting the value I wanted.
gridModel.QueryParam = "account=" + account;
gridModel.QueryParam += "&openFlag=" + openFlag;
Then to use it in the postback,
[AcceptVerbs(HttpVerbs.Post)]
public ActionResult Index(PagingParams args, string account, string openFlag)
{
Monday morning details will get you every time...