Is it really that hard though? surely the very people you're targeting are those that already complete a self assessment (I don't think pensions get paid net of PAYE do they?), so for those pensioners who a re higher rate tax payers, you just levy a 100% tax on their benefits payments.
State pensions are paid gross with no tax deducted, so yes SP is paid outside of PAYE. The govt then reduce your tax code by the amount paid as SP, and that code is then used for any/all other sources of income (private pensions, job income, etc).
You may be targeting higher paid pensioners, but you can't just dream up a tax for them. You would have to find a way of reducing their personal allowances. However reducing or otherwise fiddling with personal allowances to compensate for benefits will drag a lot of low/middle income pensoners into the Self Assessment regieme, and that is a recipe for disaster. It also has the potential to create lots of traps where earning £1 extra results in a £2 drop in income.
Your other issue is that many millionare pensioners are sensible enough to have their money in tax free shelters like ISA's or their houses. Almighty fuss if you attempt to raise tax from them.