That’s a pretty crazy way of doing things to my mind. It’s not like people generally want to travel on business, so you don’t need to use the expenses system to disincentivise it.. Can you make the the company split the payment into 4 weeks and 2 weeks at least?
I can see the per diem having to be recorded as a Benefit in Kind, as the company can’t prove it was all work based expense though - which is why reimbursement of actual accrued costs, subject to (usually challenging) company policy is probably the least bad option for the traveller..?

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