@Elliot Williams recently updated the GPIO alternative functions initialization in the STM8EF GitHub project, and a the library feature with e4thcom support make it much easier to build applications.

There is really no reason not to buy this $4.75 device if you need a programmable power supply with low noise :-)

Edit: there is at least one vendor left (the shipping cost is a bit higher but the price is still reasonable)  and the latest STM8 eForth release can be used.