No bank account on invoice when invoicing a contact

Steps to reproduce:
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1 Create a partner with a bank account. Create a contact on that partner. Note that you cannot add a bank account to the contact directly, because the bank accounts are supposed to be configured on the commercial/parent partner.

2 Create an invoice for the contact

Expected results:
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When selecting the contact, I expect the parent partner's bank account to be filled in on the invoice's 'Other Info' tab.

Actual results:
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The bank account is not added to the invoice. Moreover, when trying to select the bank account manually, the domain on partner_id prevents this. No bank account can be selected even if the contact's commercial partner has a bank account configured.

Analysis and proposed solution:
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When invoicing a contact, the onchange method considers the bank accounts of the contact, not of the commercial partner. This should be easy to fix in 7.0 in the onchange method. However, this is seems meaningless to me if no choice can be made in the case of multiple accounts anyway, due to the problem with the domain.

The bank domain cannot be properly fixed under the stable policy as it depends on having the commercial_partner_id field available in the invoice form. In trunk, applying this field in the domain should fix the problem.