Some thoughts:

* In reality I do not think the profits from seigniorage have been largely and directly transferred to taxpayers at the first place. And I do not think the fear of loss in "PV" of the seigniorage is the intuition behind the worries of taxpayers on risk asset purchase.
* Risk asset purchase by central banks does create moral hazard and unfairness in funding. Ultimately it is not the loss that matters the most, it is the re-distribution of wealth associated that matters.
* Large losses from risk asset purchase by central banks can also lead to currency depreciation.