You would have to test - but it's possible you don't care.
The 856 doesn't map the account number inbound so MA531 will just match to PO and release.
The 810 is likely the same way - it cares that it can find a PO number to match against - it's going to pull all the other information from PO20. When the invoices go out - they are going to reference Invoice and PO - it's only if the payment references account that you'll have issues.
We had 2 vendors that did this - the one just accepted the fact that we ignored it and the other ended up getting turned off EDI.