Well this is interesting ...
I have a DM250 that I bought a year ago that's been collecting dust because I couldn't get used to the UI/UX. It was imported straight from Japan so I'm pretty sure it's not the US model although I'll have to confirm that.
Customizing / tinkering with it has been difficult because it's a pretty niche product with not a ton of information online (at least on english websites).
Skimming this tutorial it seems like installing OpenBSD is a high-risk / high-reward type of thing. I'll have to look into further to see if it's worth it.
If the stock UI/UX is already a dealbreaker for you, the risk calculus is pretty different - you're basically bricking something you weren't using anyway. jcs is usually thorough about documenting the process, and OpenBSD on bare metal like this tends to be surprisingly stable once it's running.
For those considering the US model which is listed as USD 549 [1], the Japanese one currently sells for ~USD 260 in Japan [2].
[1]: https://pomera.us/products/pomera-full-suite-typewriter-for-...
[2]: https://kakaku.com/item/K0001457601
The list price appears to be ~USD 380, which is still a hefty saving compared to what they charge for the US model. There are of course software and hardware differences [3], but if it is primarily the hardware you are interested in, I see no reason not to at least consider importing one.
[3]: https://pomera.us/pages/support-help-support-pomera-dm250us-...