But the current account surplus already represents net new ownership of foreign financial assets (in this case, Eurozone-ex-Germany assets). Encouraging equity over debt is just a portfolio composition change, and would not change the current account imbalance. In particular, it would not make the macroeconomic balances of the Eurozone-es-Germany any more sustainable.

No, what is needed is a direct transfer. Currently the EU budget amounts to about 1% of EU GDP. A large fraction of the current account surplus (above a threshold of, say, 4%) of surplus states could be added to the contribution to the EU budget. If Germany doesn't like having to contribute to EU structural funds in excess of their GDP share, they can undertake domestic policies designed to reduce their current account balance below the threshold (such as, for instance, ending their domestic wage repression policy).

If you are not convinced, try it on someone who has not been entirely debauched by economics. — Piero Sraffa