Well that didn't take long did it? Within a day of Reserve announcing that the diagnostic port on the Apple Watch supported charging and that it would be updating its design to provide power over it, Apple has announced its 'Made for Apple Watch' certification program - and the diagnostic port plays no part in it.
Now that doesn't mean that Reserve - or anyone else for that matter - couldn't use the port, just that they wouldn't be able to get certification to do so. Also, Apple appears to be looking to license the design of the strap lugs, which suggests that they will be looking to shut down any third party, non-approved OEMs who look to build Watch straps outside of the programme. In fact the license's wording specifically ties licensees to 'conform in all respects to the guidelines... quality, materials, dimensions, testing and interaction with the Apple Watch lug channel'. Doesn't mean that the OEMs have to use the license, but it could cause some issues with Apple's notorious IP defences.
Those barriers are all in addition to the risk that Apple could disable charging through the port via a future software update.
The extent to which Apple seeks to control the accessory market isn't the best thing for its customers, but for OEMs seeking to innovate in design and price its a complete disaster.