iRequisition submit button pre check

We have a requirement to check if the charge accounts on a requisition being created in the Buyer Work Centre, are from different companies (first segment on the charge account being the company). I tried doing it via POR_CUSTOM_PKG but it fires either at UpdateLines or onwhile submitting the requisition, give that I have saved the requisition before submission. I need to perform the check prior to requisition is submitted.

Can anyone suggest if there is another hook or something through which this can be achieved?