I asked my other pension providers (Aviva, Friends Life, Legal & General) about the partial pension drawdowns. In each case, 25% of any single drawdown would be tax free while the rest is taxable income. In no case could the drawdowns be arranged to occur automatically; each drawdown needs to be arranged manually. (The reason offered by L&G for the inability to arrange automatic drawdowns is that government pension rules could change.)
Where providers differ is the minimum withdrawal amount and the maximum number of withdrawals per year. To summarise, including Scottish Widdows:
Minimum Maximum
Provider withdrawal withdrawals/year
Scottish Widdows £5,000 12 (1/month)
Aviva None No limit
Friends Life None 6
Legal & General £5,000 2
Aviva and Friends Life have the most liberal withdrawal policies, but I bet they amend theirs over time to be more limited.
Anywho, back on topic, taking a single payment of the same amount per year from a plan(s) is clearly possible. The HMRC's position is crystal clear: 25% of each payment is tax free, 75% is taxable.
The IRS' position is less clear, but there seems to be a general consensus (on this forum, at least) that such an approach has merit. Receipts should fall within 17.1a because they are taken as periodic pension payments, and hence would be tax free. 17.1b and 17.2 don't apply because payments are not cross-border (in my case). A Form 8833 would need to be filed. There is, however, always a risk of the IRS invoking 1.4 (saving clause) because 17.1a is not protected by 5a.