Supplier invoices needs means to deal with reimbursement of mandated operations on behalf of client (with invoice)

As very briefly mentioned here (Issue 6830: Add retention on different receivable account - Tryton issue tracker) there is a difficult case to handle for a certain category of special suppliers - namely lawyers, notaries, …

When this sort of supplier pays expenses on behalf of the client, he will put on his invoice(a) the amount paid (including VAT - which is not included in his VAT amount) and attach the corresponding invoices(b) made out to the client by this third party (of which a very frequent case may be a bailiff).
The client recuperates the VAT, where appropriate, directly on the third party invoice(b), when entered.

The invoice(a) line type that is missing currently in Tryton is something like ‘mandated payment’ and probably is simply an origin link to this/these invoice(s) recuperating the amount(s) in order to make the amount(s) payable correspond to the suppliers invoice total (by creating one or more payment lines… something like statements do)

The other way around (for client invoices) is also feasible, but a bit simpler… at least in the French plan, there is already a diverse third party debtor/creditor root account that is usually used (467x)… (the trick is to get invoicing domain rules to work there).
The amounts paid and recuperated there are always VAT included and never touch his own revenues/expenses.

Does this sound feasible?

Has anybody else already come up with a usable solution for this, or has other requirements?