I recently went to USUS headquarters in Berlin to pick up a few pen orders for some people on this forum. Although its not listed on their website, it’s possible to get any of the USUS Pi models as mechanical pencils, for an extra 10€ (so the price is 50€). The nice thing is that the mechanisms are completely interchangeable; you can convert ballpoint to MP in less than a minute. It also seems that when you buy the MP, you get a pen refill with it anyway (although I can’t confirm that that is their standard practice).
While i didn’t handle them that much, it seemed that they are well made, and the ‘knock’ mechanism is nowhere nearly as delicate as it might look.
Anyway, Hilmar (the founder) is very friendly and will probably be the one who answers your email should you decide to order from them. They’re not producing anymore but there is still some stock it seems, and they do ship internationally.