It's been a while since I ordered two cheap DC/DC converters - today I removed the LED displays to see if it's hackable.
The second one, the one that looked most promising since the seller's pictures showed ICP pads, turns out to be disappointing:
First, there are no ICP pads. Second, the voltmeter is controlled by a combination of two chips. The SOIC-8 chip is a 24C02 EEPROM. I've got no idea why they removed the label from SOIC-16 part, but it's not worth bothering since it's unlikely that a µC without EEPROM would be worth hacking.
The second one looks much better: compared to the first specimen the Nuvoton replacement for our trusty hackable STM8S almost looks like a long-lost friend. The guy who made the PCB even was nice enough to keep the option of using an STM8S003F3P6: the unpopulated pads to the left of the µC are for the Vcap capacitor!
There is more good news: S1 is connected to pin18 - PD1/SWIM. I didn't test it but ut s likey that there is nothing besides the key connected to it. Of course, there is more to figure out before I it's clear that there is still life in hackable DC/DC converters (e.g. a free-ish GPIO with PWM). If someone wants to try it, here is a recent offer for €1.25 / $1.38 (free shipping!)