The reason why I'm not sure whether my pollination attempts worked or not is that some of the flowers I haven't touched at all seem to have developed fruits anyway. I didn't pay attention before only noticing that the remains of the fruits are empty and dried out several months later. This time I'm watching them closely, and they all do seem to get bigger without withering, even though there shouldn't be any seeds inside. Here are two examples: both dorotheae and 'ventergreen' didn't have any flowering partners of their kind this year. Allthough it looks like future seed capsules, they'll probably just dry out in time, won't they?
PS: thanks to Aiko :) who shared the link to British Cactus & Succulent Society sales page, I've purchased the 2nd edition of Lithops - Treasures of the Veld by Steven Hammer. I'm so happy to finally be able to have this book on my book shelf! It's perfect to look things up if you have questions. Also, in this edition there is a new chapter about some new taxa and cultivars with pictures, so don't wait until this one is out of print as well. ;)