Espruino on your watch!

That's really neat - so you literally just power on the watch, run this app with the firmware, and you're sorted? That's the kind of thing that could easily be automated to update off-the-shelf watches.

Yes, but only for those made by DESAY INFOR TECHNOLOGY CO.,LTD. https://fccid.io/2AEMN https://www.globalsources.com/si/AS/Desay-Infor/6008849906089/Homepage.htm

So if you e.g. could source this one https://www.globalsources.com/si/AS/Desay-Infor/6008849906089/pdtl/smart-bracelet/1167004880.htm or this one https://www.globalsources.com/si/AS/Desay-Infor/6008849906089/pdtl/Heart-rate-Smart-Bracelet/1167004822.htm it would most probably work (we do have links for DFU zips for most models including this D15F or Z10F one)

EDIT:
And BTW same thing can be done for devices manufactured by "Shenzhen DO Intelligent Technology Co. Ltd" as they also use SDK11 for their ID1xx models (except that ID205 nRF52840 model) so you don't need for them to do custom firmware, you just need to meet minimum order which may be an issue too.

However if you aim for nRF52840 then SDK11 is out of question, looks like they stick with old proven SDK11 they initially started with when going for nrf52, but they cannot do this with nRF52840 .

And others possibly started with nrf52 a bit later so they stick to SDK12 (and signed firmware :-( )